State each contribution rule once

What

The contribution half of the skill states each fact once. public-writes.md is the one home of the write rule, contribute.md is a phase index, and the contradictions between the filing, monorepo, docs-MR and review leaves resolve to what the scripts do.

Why

The skill absorbed a second one, and the merge left the same rule in several places with different answers: whether vendored skills post directly, whether a partial overlap gets a new issue or a comment, clone depth against the clone script, and how many writes skip the draft recipes. A reader following one leaf could break a rule stated in another.

How

• public-writes.md names the writes outside the draft recipes and says vendored skills read and analyze but never post; the spine points there instead of counting.
• allowed-tools drops ci, check, lint and test, which would pre-approve a user's own project recipes.
• contribute.md keeps the phases and their done-when; the reviewer wave, Duo, pre-scan and dependent-MR guidance moves to review.md, the AI usage event rules to verify.md, and the squash and Changelog rule to conventions.md. Routes that need GitLab team access sit in one section.
• A partial overlap routes to a comment on the existing issue, everywhere.
• The SKILL.md park rule points at gce/operations.md § Teardown; verify.md points at gce/dev-base.md instead of restating its fixes.
• mcp.md describes the Playwright wiring as lab playwright runs it.
• The PSIRT caveat in filing.md stays.

Verification

• just ci passes, including the behavioral suite, vale in parity mode and the word budget.
• The fidelity test ran on this branch merged with the simulation and repo-level cleanups: KEEP, recorded in the follow-up MR.

The SKILL.md description is unchanged, so the routing battery does not re-run.