Fix Slack thread ownership transfer context

[gugu91]

Summary

• append explicit Slack thread transfer context (thread_ts/channel/recovery hint) to broker A2A transfer messages
• store transferred Slack channel/source metadata on threadOwnershipTransfer
• surface transferred Slack threads in Pinet inbox pointers with slack_send guidance
• hydrate follower local thread state from transfer metadata so transferred workers have channel context available for reply resolution

Fixes #756.

Investigation notes

The DB ownership transfer path was updating the broker thread owner, but the recipient-visible A2A message stayed opaque: follower polling treats agent messages separately from regular Slack inbox sync, so the worker could receive only a Pinet pointer and generic reply via pinet action=send guidance. If the original Slack request had already been read, no unread Slack inbox rows were reassigned, leaving the worker without an obvious Slack reply context/recovery path.

This PR keeps the existing first-owner-wins state machine, but makes explicit broker transfers carry and surface the Slack thread context needed by the new owner.

Validation

• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• pnpm --filter @gugu910/pi-slack-bridge test
• pre-push: pnpm lint && pnpm typecheck && pnpm test

[gugu91]

Will's reviewer agent Patch Platypus

Verdict

• Block
• The new transfer context is useful, but the follower-side hydration still preserves a stale local owner for already-cached threads.

Top findings:

• slack-bridge/helpers.ts:2212 — syncTransferredSlackThreadContexts() keeps existing?.owner instead of overwriting to the recipient's owner token. For an explicit transfer, that leaves the local cache thinking someone else still owns the thread, so reconnect/reclaim (slack-bridge/follower-runtime.ts:232) and /pinet status (slack-bridge/pinet-commands.ts:492) stay wrong.
• Please add a regression test for the existing-cached-thread/other-owner case.

[gugu91]

Will's reviewer agent Thread Newt

Re-review after 4a81820 on PR #757: Approve.

• Resolved: slack-bridge/helpers.ts:2212 now overwrites stale cached owner with agentOwner, so transferred-thread cache, reclaim, and /pinet status converge on the new owner.
• Covered: slack-bridge/helpers.test.ts:3482 adds the cached-thread/other-owner regression case.

Verification:

• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• pnpm --filter @gugu910/pi-slack-bridge test

No remaining blocking findings from the earlier stale-owner cache review.