Add browser use security policy and URL allowlisting

[kovtcharov]

Summary

Implement security controls for the browser use feature to prevent misuse, credential exposure, and unauthorized access.

Security Requirements

URL/Domain Controls

• Configurable domain allowlist (`None` = all allowed)
• Default blocked domains: banking sites, cloud consoles, email providers
• Block dangerous protocols: `file://`, `ftp://`, `data://`, `javascript://`
• Validate URLs before navigation

Resource Controls

• Max pages per session (default: 50)
• Navigation timeout (default: 30s)
• Max concurrent browser instances (default: 1)
• Memory limit per browser context
• Rate limiting on navigation (max 10 pages/min)

Content Controls

• No file downloads by default (`allow_downloads=False`)
• No access to browser storage/cookies from other sessions
• Incognito/isolated browser context per session
• Clear all browser data on session end

Logging & Audit

• Log all URLs navigated with timestamps
• Log all form interactions (field names, not values)
• Warn on navigation to login pages
• Alert if credentials detected in page content

Configuration

# In ChatAgentConfig or standalone
enable_browser: bool = False  # Opt-in only
browser_security:
  allowed_domains: null       # null = all
  blocked_domains: ["bank", "chase", "gmail"]
  blocked_protocols: ["file", "ftp", "data"]
  max_pages_per_session: 50
  allow_downloads: false
  allow_javascript: true
  incognito: true

Files

• `src/gaia/agents/chat/tools/browser_tools.py` (security validation in tool methods)
• `src/gaia/agents/chat/agent.py` (config integration)
• `tests/unit/chat/test_browser_security.py` (NEW)

Acceptance Criteria

• Blocked domains/protocols raise clear error
• Rate limiting enforced
• Browser context isolated and cleaned up
• All navigations logged
• Unit tests for every security check
• Security policy documented with rationale