feat(browser): add Lightpanda engine support with automatic Chrome fallback

Add Lightpanda as an optional browser engine for local mode.
Lightpanda is a headless browser built from scratch in Zig -- faster
navigation than Chrome with significantly less memory.

One config line to enable:
  browser:
    engine: lightpanda

New functions in browser_tool.py:
- _get_browser_engine() -- config/env reader with validation + caching
- _should_inject_engine() -- only inject in local non-cloud mode
- _needs_lightpanda_fallback() -- detect empty/failed LP results
- _chrome_fallback_screenshot() -- temporary Chrome session for screenshots
- Engine injection in _run_browser_command (--engine flag)
- browser_vision pre-routes screenshots to Chrome when engine=lightpanda

Config:
- browser.engine in DEFAULT_CONFIG (auto/lightpanda/chrome)
- AGENT_BROWSER_ENGINE in OPTIONAL_ENV_VARS
- /browser status shows engine info in local mode

Rebased from PR #7144 onto current main. All existing code preserved --
pure additions only (+520/-2).

25 new tests + 81 total browser tests pass (0 failures).

hermes_cli/config.py

@@ -550,6 +550,13 @@ DEFAULT_CONFIG = {
         "command_timeout": 30,  # Timeout for browser commands in seconds (screenshot, navigate, etc.)
         "record_sessions": False,  # Auto-record browser sessions as WebM videos
         "allow_private_urls": False,  # Allow navigating to private/internal IPs (localhost, 192.168.x.x, etc.)
+        # Browser engine for local mode.  Passed as ``--engine <value>`` to
+        # agent-browser v0.25.3+.
+        # "auto"       — use Chrome (default, don't pass --engine at all)
+        # "lightpanda" — use Lightpanda (1.3-5.8x faster navigation, no screenshots)
+        # "chrome"     — explicitly request Chrome
+        # Also settable via AGENT_BROWSER_ENGINE env var.
+        "engine": "auto",
         "auto_local_for_private_urls": True,  # When a cloud provider is set, auto-spawn local Chromium for LAN/localhost URLs instead of sending them to the cloud
         "cdp_url": "",  # Optional persistent CDP endpoint for attaching to an existing Chromium/Chrome
         # CDP supervisor — dialog + frame detection via a persistent WebSocket.
@@ -1827,6 +1834,15 @@ OPTIONAL_ENV_VARS = {
         "password": False,
         "category": "tool",
     },
+    "AGENT_BROWSER_ENGINE": {
+        "description": "Browser engine for local mode: auto (default Chrome), lightpanda (faster, no screenshots), chrome",
+        "prompt": "Browser engine (auto/lightpanda/chrome)",
+        "url": "https://github.com/vercel-labs/agent-browser",
+        "tools": ["browser_navigate", "browser_snapshot", "browser_click", "browser_vision"],
+        "password": False,
+        "category": "tool",
+        "advanced": True,
+    },
     "CAMOFOX_URL": {
         "description": "Camofox browser server URL for local anti-detection browsing (e.g. http://localhost:9377)",
         "prompt": "Camofox server URL",