docs: a contributing page for working on the system

[leeovery]

Summary

• Adds docs/contributing.md — everything in docs/ describes the product; nothing described the repository that produces it. Covers clone-to-green setup, the four local gates (and that there is no CI), rebuilding the knowledge bundle, and pointers to CLAUDE.md / CONVENTIONS.md.
• Documents the prerequisite that cost most of a session to find: frontmatter hooks in a project subagent only run once the workspace trust dialog has been accepted for the folder (2.1.218+), and until then they fail silently. /hooks shows nothing for an inactive agent, and hasCompletedProjectOnboarding can read true while trust is still false — so an old checkout looks set up and quietly isn't. Includes a one-liner to check it.
• Other known hiccups: agent definitions are cached per session, snapshot drift after engine changes is expected and how to regenerate, CHANGELOG.md is generated, shipped .sh migrations are frozen. Plus the never-pipe-a-gate rule, which produced a falsely-green report before.
• Linked from both README.md and docs/README.md.

Deliberately thin — it grows as we hit things.

Test plan

• Docs only; no behaviour changes.
• The trust-check one-liner in the page was run against this repo and returns the expected value.

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