[cc] add snapshot test for tools/list bytes (prompt-cache regression catch)

[anipotts]

Source: Apr 2026 conventions audit

Problem

cc 3.1's prompt-cache stability relies on tools/list returning byte-identical bytes across sessions. A future PR could subtly change the output (a new `description` field, reordered properties, schema dialect bump) and silently invalidate every prompt-cache entry that includes the cc tool. There's no test catching this.

Fix

`tests/snapshot.test.ts` that:

1. Spawns the bun MCP server via stdio
2. Sends initialize + tools/list
3. Hashes the response and compares to a checked-in fixture (`tests/fixtures/tools-list.sha256`)
4. Fails on hash mismatch with a clear "regenerate fixture" message

Bonus

Also snapshot:

• `initialize` response (includes instructions field)
• A canonical `cc(action='check')` empty-state response

Acceptance criteria

• Test fails when tool description / schema changes
• Failure message includes the diff and the regen command
• CI runs the snapshot test on every PR

[github-actions]

@claude please triage this issue per .github/AI_REVIEW_RUBRIC.md (issue-triage section).