docs: augment agent platform architecture plan + task breakdown

- Augmented 2026-03-15-agent-platform-architecture.md with 6 missing
  sections: Teams Architecture, REST Route Specifications, /provider
  OAuth flow (URL+clipboard), preferences mutable migration, Test
  Strategy (per-task), and Phase Execution Order (wave plan)
- Created spin-off plan stubs: gatekeeper-service.md,
  task-queue-unification.md, chroot-sandboxing.md
- Added P8-007 through P8-019 to TASKS.md (13 new tasks)
- Created Gitea issues #160-#172, Phase 8 milestone ms-165
- Updated MISSION-MANIFEST.md (Phase 8 in-progress)
- Updated scratchpad with session 14 decisions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/MISSION-MANIFEST.md

@@ -39,7 +39,7 @@
 | 5   | ms-162 | Phase 5: Remote Control (v0.0.6)        | done        | —      | #99   | 2026-03-14 | 2026-03-14 |
 | 6   | ms-163 | Phase 6: CLI & Tools (v0.0.7)           | done        | —      | #104  | 2026-03-14 | 2026-03-14 |
 | 7   | ms-164 | Phase 7: Feature Completion (v0.0.8)    | done        | —      | —     | 2026-03-15 | 2026-03-15 |
-| 8   | ms-165 | Phase 8: Polish & Beta (v0.1.0)         | not-started | —      | —     | —          | —          |
+| 8   | ms-165 | Phase 8: Polish & Beta (v0.1.0)         | in-progress | —      | —     | 2026-03-15 | —          |
 
 ## Deployment