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/native_host.py

@@ -47,13 +47,23 @@ PAGEABLE_COMMANDS = {
 
 # --- Native Messaging protocol (4-byte LE length prefix + UTF-8 JSON) ---
 
+def _read_exact_stream(stream, n: int) -> bytes | None:
+    buf = b""
+    while len(buf) < n:
+        chunk = stream.read(n - len(buf))
+        if not chunk:
+            return None  # real EOF
+        buf += chunk
+    return buf
+
+
 def read_native_message(stream) -> dict | None:
-    raw_len = stream.read(4)
-    if len(raw_len) < 4:
+    raw_len = _read_exact_stream(stream, 4)
+    if raw_len is None:
         return None
     msg_len = struct.unpack("<I", raw_len)[0]
-    data = stream.read(msg_len)
-    if len(data) < msg_len:
+    data = _read_exact_stream(stream, msg_len)
+    if data is None:
         return None
     return json.loads(data.decode("utf-8"))
 
@@ -125,10 +135,16 @@ def stdin_reader(alias: str):
 def socket_server(sock_path: str, bound_sock: "socket.socket | None" = None):
     if is_windows():
         while True:
+            listener = None
             try:
                 listener = Listener(sock_path, family="AF_PIPE")
                 conn = listener.accept()
             except OSError:
+                if listener is not None:
+                    try:
+                        listener.close()
+                    except Exception:
+                        pass
                 break
             threading.Thread(target=handle_cli_connection, args=(conn, listener), daemon=True).start()
         return