Add public Board demo rehearsal

[jeffhuber]

Summary

• add examples/board-demo/ with a public synthetic Board rehearsal
• include sample board-events.jsonl and reviewer-spend.json fixtures for local Board history, verdict, and spend panels
• link the demo from README and quickstart, with release-hygiene coverage for discoverability and privacy safety

Privacy

The demo contains only synthetic metadata. It excludes source, raw diffs, transcripts, issue body text, raw stdout/stderr, auth output, browser history, local paths, local secret values, and secrets.

Verification

• scripts/dev-python -m code_mower.cli board events --store-path examples/board-demo/board-events.jsonl
• scripts/dev-python -m code_mower.cli board doctor --repo example/widget-service --store-path examples/board-demo/board-events.jsonl --spend-path examples/board-demo/reviewer-spend.json --json
• scripts/dev-python -m pytest tests/test_release_hygiene.py -q -k 'public_board_demo_artifacts_are_parseable_and_sanitized or privacy_scan_is_clean'
• scripts/dev-python scripts/privacy_scan.py
• scripts/dev-python -m ruff check tests/test_release_hygiene.py
• scripts/dev-python -m pytest tests/test_board.py -q
• scripts/dev-python -m pytest tests/test_release_hygiene.py -q -k 'public_board_demo_artifacts_are_parseable_and_sanitized or board_data_contract_preserves_local_only_cloud_boundary'
• scripts/dev-python -m code_mower.checks pr-size-lint --repo-path . --base-ref main --max-changed-lines 300
• scripts/dev-python -m pytest -q (826 passed, 439 subtests passed)

Closes #529.

[gitar-bot]

Code Review ✅ Approved

Adds public Board demo rehearsal with synthetic board-events.jsonl and reviewer-spend.json fixtures, linked from README and quickstart for discoverability. Demo contains only synthetic metadata with privacy-sensitive content excluded. All verification checks passed, including release hygiene and privacy scans. No issues found.

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[jeffhuber]

Claude audit (merge-authority lane)

Head SHA: 7471d7ed814f5150df127c5d611103233be0379b
Findings: P0=0, P1=0, P2=0, P3=0 (blocker policy: any P0/P1/P2 -> BLOCKED)

Claude Audit: PASS

Summary:

Additive docs/example PR (board-demo fixtures, README/quickstart links, matching test). Fixtures are synthetic and consistent with claimed schemas, privacy boundary claims hold (no real paths/secrets), and the new test exercises existing board/board_store code against the new fixtures. No correctness, security, or contract issues found.

Findings: none.