fix(plugin): handoff requests spec changes instead of editing the package

[chrispezza]

Summary

Closes #71. Corrects the handoff text shipped in #69.

• The flaw. The handoff told the implementation agent to update the spec files once the builder agreed. /compile deletes the package and rebuilds it from the source artifacts, so those edits were never validated and were lost on the next compile.
• The fix. After agreement the agent edits nothing under the package. It writes spec-change-request.md with the exact YAML per spec file and the source artifact each belongs in, asks the builder to re-run /compile, and builds the feature from the recompiled package.
• The handoff now carries the spec-file-to-artifact table. A test keeps it equal to the compiler's own map.
• Plugin version 0.5.2 to 0.5.3. Example handoff regenerated.

Evidence

Two batches of six isolated headless two-turn runs (Sonnet 5.5, tea-tracker package, a spec-silent is_favorite request, then "Yes, add it. Go ahead.").

	Handoff from #69	This PR
Turn 1: withheld the field and asked	6 of 6	6 of 6
Turn 2: edited files under the package	6 of 6	0 of 6
Edited package still validates	3 of 6	not applicable
Turn 2: wrote the change request only	not applicable	6 of 6
Built the field before a recompile	6 of 6	0 of 6

Limits: one fixture, one model, one request. The loop was not closed with a recompiled package.

Behaviour change for plugin users

A feature outside the spec now waits for the builder to edit the source artifacts and recompile. Slower than an in-place edit, and the only path on which the change is validated and survives.

Test plan

• python -m unittest discover -s checks/tests — 34 tests pass
• python checks/run_checks.py --ci --require-validator — 0 findings
• Fixture package still validates
• CI green

ADR impact

• Existing ADR amended by an appended, dated section: ADR 0010

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