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).

.env.example

@@ -244,6 +244,15 @@ BROWSERBASE_PROXIES=true
 # Uses custom Chromium build to avoid bot detection altogether
 BROWSERBASE_ADVANCED_STEALTH=false
 
+# Browser engine for local mode (default: auto = Chrome)
+# "auto"       — use Chrome (don't pass --engine flag)
+# "lightpanda" — use Lightpanda (1.3-5.8x faster navigation, no screenshots)
+# "chrome"     — explicitly request Chrome
+# Requires agent-browser v0.25.3+. Lightpanda commands that fail or return
+# empty results are automatically retried with Chrome.
+# Also configurable via browser.engine in config.yaml.
+# AGENT_BROWSER_ENGINE=auto
+
 # Browser session timeout in seconds (default: 300)
 # Sessions are cleaned up after this duration of inactivity
 BROWSER_SESSION_TIMEOUT=300