feat(github): add Mattermost self-heal workflow

[Dustymon111]

Summary

• add an allowlisted Mattermost WebSocket bot with guarded GitHub Actions dispatch
• add a dev-locked self-heal workflow that validates issues and creates isolated self-heal PRs
• add deterministic self-heal branch support and parser coverage

Verification

• bun run --cwd packages/mattermost test
• bun run --cwd packages/mattermost typecheck
• pre-push repository-wide typecheck