Make triage bot changesets non-optional

[matthewp]

Changes

Triage-bot PRs frequently ship without a changeset. Root cause is in the skill instructions, which treat the changeset as a discretionary step the LLM can rationalize away for "trivial" fixes.

• triage/fix.md Step 9: reframed from "only do this if..." to required for any successful fix that modifies a package, with an explicit guard that one-line/type-only fixes still need one. Inlined the pnpm changeset --empty commands so no second skill-load is needed.
• triage/fix.md Output: removed the "if no changeset was created, explain why" escape hatch.
• triage/fix.md Cleanup: added a gate — the final git status must include exactly one new .changeset/*.md.
• astro-pr-writer: backstop turned from passive ("check it exists") to active ("create it now if missing; don't post without one").

Testing

• None. Instruction-only change to agent skill markdown; no package code touched.

Docs

• No docs update needed. Internal agent-skill instructions, not user-facing.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0239835

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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