Sprint 18 ceremony: recall

[laynepenney]

Summary

Sprint 18 ceremony PR for recall. Merges all sprint-18 work into main with merge conflicts resolved.

Sprint 18 deliverables (recall)

• recall#672: fix: SQLite lock contention in channel reap path (real regression fix)
• recall#668: feat: channel seam hooks for premium coordination gating
• recall#670: test: OSS protocol definitions for org + entitlements seam
• recall#553: channel seam hooks implementation
• Blog index conflict resolved (sprint-18 curated index merged with main's sprint 8-11 recap cards)

Merge conflict resolution

• docs/blog/index.html: Combined sprint-18's curated blog index with main's Sprint 8-11 recap cards. Sprint 13 positioned correctly in chronological order. Author image styles normalized.

Premium boundary: core OSS (recall channel infrastructure, protocol definitions).

Test plan

• All sprint-18 tests pass (169 passed, 1 skipped)
• Merge conflict cleanly resolved
• grip#541 and premium#597 ceremony PRs already merged

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