fix(slackbot): route thread mentions through DM

[samuelclay]

Summary

• route addressed Watch Agent mentions inside existing channel threads into a DM thread with links back to the original request and source thread
• keep explicit inline requests in the original Slack thread
• let routed DM users approve posting the latest successful result back to the original thread
• add HTTP-level and helper coverage for routing, approval publishing, and Centaur handoff metadata

Verification

• bun test src/.test.ts src/**/.test.ts
• bun tsgo --project tsconfig.json --noEmit
• bun run lint (passes with existing warnings; oxlint --fix-dangerously has an unrelated bad autofix in agent-session.test.ts, reverted locally)
• git diff --check
• docker build -t centaur-slackbot-thread-dm-routing:local -f services/slackbot/Dockerfile .
• GitHub CI / Publish Images / PR Review Notifications are green on head 3f8db0e5

Notes

• Could not run the documented local just build-one slackbot / just deploy flow because just is not installed and the current local Kubernetes context cannot list API resources.
• Centaur is public, so the copied internal Tavus-Engineering/pr-changelog-action workflow could not resolve. I replaced the main workflow with a public-repo-safe GitHub Script implementation.
• The PR notification workflow now runs successfully but currently skips Slack posting until CVI_BOT_SLACK_TOKEN is configured as a GitHub Actions secret on Tavus-Engineering/centaur.