feat(phase31): add web_fetch, high-speed grep_search, and filesystem checkpoints with /undo

[De-pitcher]

Summary

• Direct Web & Documentation Fetcher (\web_fetch):
  • Direct HTTP(S) fetching with User-Agent, bounded byte offsets, and domain safety filters.
  • Zero-dependency \HTMLToMarkdownConverter\ preserving headers, code blocks, lists, and links while stripping styles and scripts.
• High-Speed Regex & Text Grep (\grep_search):
  • Multi-file regex and literal search across all files with line numbers and snippets.
  • Accelerated by native ripgrep (
    g) when present on PATH, with an in-process zero-dependency streaming Python regex fallback.
• Filesystem Checkpoints & Atomic Rollback (/undo):
  • Added \CheckpointManager\ taking automatic turn snapshots of mutated and created files.
  • Added /undo, /undo diff, and /undo list\ slash commands across interactive chat CLI and full-screen TUI dashboard.
• Registration & Tool Schemas:
  • Registered \SubAgentType.WEB_FETCH\ and \SubAgentType.GREP_SEARCH\ in \�gentcli/agent/registry.py, \�gentcli/subagents/init.py, and \�gentcli/tools_schema.py.
• Testing & Quality Gates:
  • Added dedicated test suite \ ests/test_phase31_vital_tools.py\ with 100% pass rate.
  • 0 lint errors (
    uff) and 0 type errors across 103 source files (\mypy).