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).

cli.py

@@ -298,6 +298,7 @@ def load_cli_config() -> Dict[str, Any]:
         "browser": {
             "inactivity_timeout": 120,  # Auto-cleanup inactive browser sessions after 2 min
             "record_sessions": False,  # Auto-record browser sessions as WebM videos
+            "engine": "auto",  # Browser engine: auto (Chrome), lightpanda, chrome
         },
         "compression": {
             "enabled": True,      # Auto-compress when approaching context limit
@@ -7131,7 +7132,20 @@ class HermesCLI:
                 if provider is not None:
                     print(f"🌐 Browser: {provider.provider_name()} (cloud)")
                 else:
-                    print("🌐 Browser: local headless Chromium (agent-browser)")
+                    # Show engine info for local mode
+                    try:
+                        from tools.browser_tool import _get_browser_engine
+                        engine = _get_browser_engine()
+                    except Exception:
+                        engine = "auto"
+                    if engine == "lightpanda":
+                        print("🌐 Browser: local Lightpanda (agent-browser --engine lightpanda)")
+                        print("   ⚡ Lightpanda: faster navigation, no screenshot support")
+                        print("   Automatic Chrome fallback for screenshots and failed commands")
+                    elif engine == "chrome":
+                        print("🌐 Browser: local headless Chrome (agent-browser --engine chrome)")
+                    else:
+                        print("🌐 Browser: local headless Chromium (agent-browser)")
             print()
             print("   /browser connect      — connect to your live Chrome")
             print("   /browser disconnect   — revert to default")