feat(slack): support native agent streaming

[MiscMich]

Summary

• publish agent thinking status and reply text through Slack's native agent APIs
• render durable QM tasks as native timeline updates without exposing reasoning or tool arguments
• preserve the existing reaction, Block Kit, and final-message delivery path when native streaming is unavailable
• delete an uncheckpointed stream before falling back so recovery cannot duplicate a reply
• let the seeded GitHub/GitLab skill use a per-user VAULT_TOKEN_API_GITHUB_COM as command-scoped GH_TOKEN, without printing or persisting it

Verification

• npm run typecheck
• npm run lint
• npm run lint:ox
• npm run format:check
• focused Slack presenter, integration, delivery, messaging, turn-stream, and seeded-skill tests
• latest seeded-skill suite: 11/11 passed

Live proof

• Slack persisted the native agent reply with streaming_state: completed
• the isolated sandbox executed command -v gh and gh --version and returned /usr/bin/gh / gh 2.97.0
• no GitHub connector token was present, proving that no host credential was silently inherited
• the final Slack UI rendered the response as an AGENT reply in the DM thread
• the prompt explicitly prohibited authentication and external mutations

---