feat: Ed25519 challenge-response auth + YubiKey/SSH agent support (v0.9.0)

Security:
- serve.py: server now sends nonce challenge before accepting any command;
  clients sign nonce + SHA256(canonical_payload) with Ed25519 key
- New --authorized-keys FILE option for serve; token auth still works as fallback
- Connection limit: BoundedSemaphore(64) in serve.py
- Secure file creation with os.open(..., 0o600) for token/key files
- New auth.py module: keygen, file key load/save, SSH agent protocol (pure Python),
  sign/verify helpers compatible with both file keys and agent-held keys (YubiKey,
  TPM, gpg-agent)

Features:
- YubiKey support via SSH agent protocol — no new runtime deps, just $SSH_AUTH_SOCK
- New `browser-cli auth` command group: keygen, trust, show, keys
- Global --key PATH flag (or BROWSER_CLI_KEY env) selects signing key;
  pass "agent" or "agent:<selector>" to use SSH agent key
- BrowserCLI Python API gains key= parameter

Bug fixes (11 issues across two review passes):
- client.py: check response is not None before json.loads
- native_host.py: _read_exact_stream loop handles EINTR short reads; fix Windows
  Listener leak on accept error
- __init__.py: open_wait / tabs_watch_url raise RuntimeError instead of silent None
- extension/tabs.ts: dedupe skips tabs without URL; tabsSort uses pendingUrl fallback
- extension/session.ts: removeListener before addListener prevents duplicate handlers

Breaking: TCP serve protocol now sends a challenge frame first (v0.9.0)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

browser_cli/__init__.py

@@ -18,6 +18,7 @@ Usage:
 """
 from collections.abc import Callable, Iterable
 from dataclasses import dataclass
+from pathlib import Path
 
 from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
 from browser_cli.models import Group, Tab
@@ -33,7 +34,7 @@ class BrowserCounts:
 
 
 class BrowserCLI:
-    def __init__(self, browser: str | None = None, remote: str | None = None, token: str | None = None):
+    def __init__(self, browser: str | None = None, remote: str | None = None, token: str | None = None, key: str | None = None):
         """
         Args:
             browser: Profile alias to target. Required when multiple browser
@@ -42,14 +43,18 @@ class BrowserCLI:
                     Format: ``"host:port"`` (e.g. ``"192.168.1.10:8765"``).
                     Can be combined with ``browser`` to route to a specific
                     remote profile.
-            token: Auth token for the remote serve instance.
+            token: Auth token for the remote serve instance (legacy token auth).
+            key: Path to Ed25519 private key PEM for pubkey auth. When set,
+                 overrides token auth. Defaults to ``~/.config/browser-cli/client.key.pem``
+                 if that file exists.
         """
         self._browser = browser
         self._remote = remote
         self._token = token
+        self._key = Path(key) if key else None
 
     def _cmd(self, command: str, args: dict | None = None):
-        return send_command(command, args, profile=self._browser, remote=self._remote, token=self._token)
+        return send_command(command, args, profile=self._browser, remote=self._remote, token=self._token, key=self._key)
 
     def _multi_browser_targets(self):
         if self._browser is not None:
@@ -64,10 +69,11 @@ class BrowserCLI:
 
     def _collect_multi_browser(self, command: str, args: dict | None = None):
         results = []
-        for target in self._multi_browser_targets():
+        targets = self._multi_browser_targets()
+        for target in targets:
             try:
                 if target.remote:
-                    data = send_command(command, args, profile=target.profile, remote=target.remote, token=target.token)
+                    data = send_command(command, args, profile=target.profile, remote=target.remote, token=target.token, key=self._key)
                 else:
                     data = send_command(command, args, profile=target.profile)
             except (BrowserNotConnected, RuntimeError):
@@ -75,7 +81,7 @@ class BrowserCLI:
             results.append((target, data))
         if results:
             return results
-        if self._multi_browser_targets():
+        if targets:
             raise BrowserNotConnected(
                 "Cannot resolve a browser socket automatically.\n"
                 "Make sure the browser is running with the browser-cli extension enabled,\n"
@@ -173,7 +179,9 @@ class BrowserCLI:
             "url": url, "timeout": int(timeout * 1000),
             "background": background, "window": window, "group": group,
         })
-        return self._make_tab(data) if isinstance(data, dict) and "id" in data else data
+        if not isinstance(data, dict) or "id" not in data:
+            raise RuntimeError("navigate.open_wait returned unexpected data")
+        return self._make_tab(data)
 
     def wait_for_load(
         self,
@@ -291,7 +299,9 @@ class BrowserCLI:
     ) -> "Tab":
         """Block until the tab URL matches regex pattern. Returns the Tab."""
         data = self._cmd("tabs.watch_url", {"pattern": pattern, "tabId": tab_id, "timeout": int(timeout * 1000)})
-        return self._make_tab(data) if isinstance(data, dict) and "id" in data else data
+        if not isinstance(data, dict) or "id" not in data:
+            raise RuntimeError("tabs.watch_url returned unexpected data")
+        return self._make_tab(data)
 
     def tabs_screenshot(
         self,
@@ -440,7 +450,7 @@ class BrowserCLI:
                 for target, windows in multi_results
                 for window in (windows or [])
             ]
-        return self._cmd("windows.list", {})
+        return self._cmd("windows.list", {}) or []
 
     def windows_rename(self, window_id: int, name: str) -> None:
         self._cmd("windows.rename", {"windowId": window_id, "name": name})
@@ -450,12 +460,12 @@ class BrowserCLI:
 
     def windows_open(self, url: str | None = None) -> dict:
         """Open a new browser window, optionally on a URL."""
-        return self._cmd("windows.open", {"url": url})
+        return self._cmd("windows.open", {"url": url}) or {}
 
     # ── DOM ───────────────────────────────────────────────────────────────
 
     def dom_query(self, selector: str) -> list[dict]:
-        return self._cmd("dom.query", {"selector": selector})
+        return self._cmd("dom.query", {"selector": selector}) or []
 
     def dom_click(self, selector: str) -> None:
         self._cmd("dom.click", {"selector": selector})
@@ -464,13 +474,13 @@ class BrowserCLI:
         self._cmd("dom.type", {"selector": selector, "text": text})
 
     def dom_attr(self, selector: str, attr: str) -> list[str]:
-        return self._cmd("dom.attr", {"selector": selector, "attr": attr})
+        return self._cmd("dom.attr", {"selector": selector, "attr": attr}) or []
 
     def dom_text(self, selector: str) -> list[str]:
-        return self._cmd("dom.text", {"selector": selector})
+        return self._cmd("dom.text", {"selector": selector}) or []
 
     def dom_exists(self, selector: str) -> bool:
-        return self._cmd("dom.exists", {"selector": selector})
+        return self._cmd("dom.exists", {"selector": selector}) or False
 
     def dom_scroll(self, selector: str | None = None, *, x: int | None = None, y: int | None = None) -> None:
         """Scroll to a CSS selector or to pixel coordinates."""
@@ -630,13 +640,13 @@ class BrowserCLI:
     # ── Extract ───────────────────────────────────────────────────────────
 
     def extract_links(self) -> list[dict]:
-        return self._cmd("extract.links", {})
+        return self._cmd("extract.links", {}) or []
 
     def extract_images(self) -> list[dict]:
-        return self._cmd("extract.images", {})
+        return self._cmd("extract.images", {}) or []
 
     def extract_text(self) -> str:
-        return self._cmd("extract.text", {})
+        return self._cmd("extract.text", {}) or ""
 
     def extract_json(self, selector: str):
         return self._cmd("extract.json", {"selector": selector})
@@ -653,7 +663,7 @@ class BrowserCLI:
         self._cmd("session.load", {"name": name})
 
     def session_diff(self, name_a: str, name_b: str) -> dict:
-        return self._cmd("session.diff", {"nameA": name_a, "nameB": name_b})
+        return self._cmd("session.diff", {"nameA": name_a, "nameB": name_b}) or {}
 
     def session_list(self) -> list[dict]:
         """Return saved sessions.
@@ -667,7 +677,7 @@ class BrowserCLI:
                 for target, sessions in multi_results
                 for session in (sessions or [])
             ]
-        return self._cmd("session.list", {})
+        return self._cmd("session.list", {}) or []
 
     def session_remove(self, name: str) -> None:
         self._cmd("session.remove", {"name": name})