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>

pyproject.toml

@@ -1,10 +1,11 @@
 [project]
 name = "browser-cli"
-version = "0.8.7"
+version = "0.9.0"
 description = "Control your real running browser from the terminal via a browser extension"
 requires-python = ">=3.10"
 dependencies = [
     "click>=8",
+    "cryptography>=42",
     "rich>=13",
 ]