feat: add Oh My Pi (OMP) platform support with native extension hook

[dull-bird]

This PR adds first-class support for the Oh My Pi (OMP) coding harness.

What changed

• Added an omp platform entry that installs the graphify skill to the .agents/skills/graphify/SKILL.md location.
• Implemented a native OMP extension (graphify.ts) that hooks tool_result events:
  • Detects Bash search commands (grep, rg, find, fd, ...) and prepends a nudge to use graphify query.
  • Detects Read/Glob on source files and prepends a nudge to use graphify query/explain/path.
  • Only activates when graphify-out/graph.json exists.
• Extension is written to user scope (~/.omp/agent/extensions/graphify.ts) for global installs and project scope (./.omp/extensions/graphify.ts) for project installs.
• Added graphify omp install / graphify omp uninstall CLI commands, including --project support.
• Updated uninstall_all to clean up OMP extensions and skill files.
• Updated _agents_install to display Oh My Pi (OMP) and skip the no-hook note since OMP uses a native extension hook.
• Updated README platform tables, hook-platform note, and command reference.
• Added omp / oh-my-pi keywords and the platform name to pyproject.toml.

Testing

• Ran uv run pytest tests/test_install*.py: 165 passed, 1 skipped.
• Manually tested global and project install/uninstall; verified extension files are written and removed correctly.

Notes

• OMP auto-discovers extensions from ~/.omp/agent/extensions and <cwd>/.omp/extensions, so no settings-file registration is required.
• The extension is intentionally advisory (nudges are prepended to tool results without blocking or modifying tool execution).