docs(groundskeeper): document every manifest field and link the worked example

Andrew Dunnrequested to merge
docs/groundskeeper-keeper-reference into main

Summary

Adds a keeper reference to the README: the envelope, the guardrails block, comment keys, and one field table per keeper kind, covering every field the engine accepts with its type, required or optional, default, constraints, and refusal preconditions. Review found five of six keeper kinds had no documented fields, and a manifest written from the README alone failed validation repeatedly. Also links tests/groundskeeper/fixtures/manifest.json, the complete worked example that was sitting unreferenced.

The tables are hand-written against load_manifest() and _validate_keeper(), not generated, so a future schema change can drift from them; tests/groundskeeper/validate-manifests.py remains the executable check. Documentation only, no behavioural change.

Test plan

  • bash tests/groundskeeper/run.sh green and unchanged (250 passed)
  • python3 tests/groundskeeper/validate-manifests.py green over both repository fixtures
  • Every documented field traced to its check in scripts/groundskeep.py

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