docs(standards): name a root CODING_STANDARDS.md and CONTRIBUTING.md as rung-4 standards sources #12141

Parent #7584 records "Standards discovery names a root CODING_STANDARDS.md and CONTRIBUTING.md as rung-4 sources (Q523)". The owner accepted this on 2026-10-15 as part of the topic-10 delivery-chain list, where the source is Matt Pocock's code-review skill, which reads those files. Rung 4 of the standards resolution ladder (docs/contracts/standards/README.md:200-205) infers standards from repository context when no index exists, and today it names docs directories, ecosystem configs and a location declared in CLAUDE.md, but neither file. scripts/shared-copies.txt copies that contract into plugins/planning/reference/standards-contract.md, plugins/review/reference/standards-contract.md and plugins/improvement/reference/standards-contract.md.

Done when

• Rung 4 of "Resolution ladder" in docs/contracts/standards/README.md names a CODING_STANDARDS.md and a CONTRIBUTING.md at the resolution root as inference sources, alongside the sources it already names.
• The three copies in plugins/planning/reference/, plugins/review/reference/ and plugins/improvement/reference/ match the canonical file, regenerated by scripts/sync-shared-copies.sh. CI's copy check passes.
• No other rung changes, and the "No silent writes" rule after the ladder is unchanged.
• .changes/planning/*.md, .changes/review/*.md and .changes/improvement/*.md fragments with bump: patch describe the change.
• The PR's CI is green.

Out of scope

• The per-concern standards layout (standards: per-concern standards layout #7583).

Parent: #7584