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/version_manager.py

@@ -0,0 +1,17 @@
+from importlib.metadata import version as _pkg_version
+
+PROTOCOL_MIN_CLIENT = "0.9.0"
+
+
+def parse_version(v: str) -> tuple[int, ...]:
+    try:
+        return tuple(int(x) for x in v.lstrip("v").split("."))
+    except ValueError:
+        return (0,)
+
+
+def get_installed_version() -> str:
+    try:
+        return _pkg_version("browser-cli")
+    except Exception:
+        return "0.0.0"