Enforce channel message ownership

[yourbuddyconner]

Summary

• add a durable channel message ownership index with tombstones and admin override
• inject a Worker-owned capability into actions and channel routing
• enforce Slack update/delete ownership and register all routed outbound messages
• document the boundary and delivery-uncertain behavior

Verification

• pnpm typecheck
• pnpm test (158 files, 1828 tests)
• targeted ownership, Slack, router, webhook, and Slack-event tests
• final code review approved