feat: token-auth removal, security hardening, Stripe-style compat layer (v0.9.2)

- Remove token auth entirely; only Ed25519 pubkey auth or --no-auth
- Add 32 MB message-size cap in serve and client (DoS protection)
- Set Unix socket to 0o600 after bind in native_host (multi-user hardening)
- Enforce browser-cli/VERSION user-agent on all TCP connections
- Add PROTOCOL_MIN_CLIENT check (>= 0.9.0) server- and client-side
- Include server_version + min_client_version in challenge frame
- Add browser_cli/version_manager.py: parse_version, get_installed_version
- Add browser_cli/compat.py: Stripe-style versioning layer with adapt_request
  / adapt_response hooks; baseline 0.9.2, no shims needed yet
- Fix BrowserCLI key handling: no Path() wrap for agent specs
- Fix _multi_browser_targets() to forward key to remote_browser_targets()

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

browser_cli/native_host.py

@@ -156,6 +156,7 @@ def socket_server(sock_path: str, bound_sock: "socket.socket | None" = None):
             path.unlink()
         sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
         sock.bind(sock_path)
+        os.chmod(sock_path, 0o600)
         sock.listen(16)
 
     while True:
@@ -319,6 +320,7 @@ def main():
             path.unlink()
         bound_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
         bound_sock.bind(sock_path)
+        os.chmod(sock_path, 0o600)
         bound_sock.listen(16)
     else:
         bound_sock = None