fix(slack): add dispatcher message update action

[gugu91]

Summary

• Fixes Slack dispatcher should support updating existing messages #730.
• Adds a compact Slack dispatcher update action backed by Slack chat.update.
• Requires explicit channel and message ts, plus text and/or replacement Block Kit blocks.
• Exposes action-specific help/schema/examples with guardrail name slack:update.
• Updates README and bundled Slack bridge skill quick-map/docs.

Token-footprint / progressive discovery

The action is added to the existing compact Slack dispatcher catalog only by name/summary. Detailed args remain discoverable via slack action=help args.topic=update, preserving progressive discovery and avoiding a new hot-path tool schema.

Validation

• pnpm --filter @gugu910/pi-slack-bridge test -- slack-tools.test.ts
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm lint && pnpm typecheck && pnpm test
• pre-push hook reran pnpm lint && pnpm typecheck && pnpm test

No merge performed.

[gugu91]

Local code-reviewer result: Block.

Finding: slack:update confirmation context is wrong for threaded replies. The new update action passes the target message ts into the guardrail/confirmation policy, but confirmations are stored/consumed by Slack thread root thread_ts. For a reply update, approval in the visible thread would not match the later retry.

Required follow-up before ready:

• Add optional thread_ts / approval-context input to the update action.
• Use that value for requireToolPolicy; keep ts only for Slack chat.update.
• Add a regression for updating a threaded reply behind security.requireConfirmation.

No merge recommended until fixed.