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>

tests/test_cli.py

@@ -168,7 +168,7 @@ def test_clients_reads_registry_with_trailing_garbage(tmp_path):
     assert "0.8.2" in result.output
 
 def test_clients_remote_uses_remote_endpoint_without_local_registry():
-    def fake_send_command(command, args=None, profile=None, remote=None, token=None):
+    def fake_send_command(command, args=None, profile=None, remote=None, token=None, key=None):
         assert command == "clients.list"
         assert profile is None
         assert remote == "127.0.0.1:8765"
@@ -194,7 +194,7 @@ def test_clients_remote_respects_global_browser_route():
         result = CliRunner().invoke(main, ["--remote", "127.0.0.1:8765", "--browser", "work", "clients"])
 
     assert result.exit_code == 1
-    send_command.assert_called_once_with("clients.list", profile="work", remote="127.0.0.1:8765", token=None)
+    send_command.assert_called_once_with("clients.list", profile="work", remote="127.0.0.1:8765", token=None, key=None)
 
 
 def test_clients_browser_alias_resolves_to_remote():
@@ -211,7 +211,7 @@ def test_clients_browser_alias_resolves_to_remote():
     )
     all_remote_targets = [resolved_target]
 
-    def fake_send_command(command, args=None, profile=None, remote=None, token=None):
+    def fake_send_command(command, args=None, profile=None, remote=None, token=None, key=None):
         assert command == "clients.list"
         assert profile == "automatisation"
         assert remote == "192.168.188.104:8765"