reviewer.md の checkpoint warning 抑制と postmortem 誤検出防止

[clonable-eden]

closes #519

Summary

• spawn.sh の checkpoint 存在チェックを AGENT_TYPE == worker の場合のみに限定。Reviewer は --resume で起動するが SUSPEND/checkpoint は Worker 専用概念のため、warning は不要
• postmortem-patterns.md の「Missing checkpoint file」パターンに Exclusion を追加。Reviewer spawn 時の checkpoint 不在は正常動作として除外対象に

Test Plan

• test-spawn-checkpoint-warning.sh — checkpoint warning が AGENT_TYPE で制御されることを検証
• test-spawn-resume.sh — 既存の resume テストがパスすることを確認
• test-spawn-reviewer.sh — 既存の reviewer テストがパスすることを確認

[clonable-eden]

Review: APPROVE ✅

Summary

Changes are well-scoped, correct, and aligned with the issue requirements.

Correctness

• spawn.sh: Checkpoint check correctly gated behind AGENT_TYPE == "worker". Reviewers using --resume no longer emit spurious warnings. The warning is preserved for Workers where it remains meaningful.
• postmortem-patterns.md: Exclusion is clear, well-placed under the "Missing checkpoint file" pattern, and accurately describes why the message should be excluded.

Conventions

• Follows UNIX principles: Silence (suppress irrelevant noise), Repair (warning preserved where meaningful), Clarity (comments explain the rationale).
• $((var + 1)) pattern used correctly in tests (bash 3.2 safe).
• PR body includes closes #519. Conventional commit format followed.

Tests

• Content-based tests on spawn.sh are a pragmatic approach given spawn.sh's heavy side effects (git worktree, claude --resume). Both tests verify the structural guard is in place.
• CI: all 4 checks pass.

Scope

Focused exactly on the two files identified in the issue. No unrelated changes.

Note: Submitted as COMMENT because PR author and reviewer share the same GitHub identity. Verdict is APPROVE.