Extract TeamToolBackend interface from concrete *TeamToolManager to enable
unit testing of the 17 team task action handlers without DB or message bus.
- Add TeamToolBackend interface (19 methods) in team_tool_backend.go
- Add exported wrappers on TeamToolManager (cache, helpers, dispatch, manager)
- Migrate 133 call sites across 5 action handler files (pure casing rename)
- Change TeamTasksTool.manager field from *TeamToolManager to TeamToolBackend
- Add mock backend infrastructure (baseNoopTeamStore + mockTaskStore + mockBackend)
- Add 94 unit subtests across 6 test files covering all actions:
lifecycle (claim/complete/cancel/review/approve/reject),
create validation, blocker escalation, mutations (comment/progress/attach/update),
followup (ask_user/clear_ask_user/retry), read actions (notification/policy/list/search/get)
WorkspaceInterceptor and PostTurnProcessor remain on concrete *TeamToolManager.
No behavioral changes — all existing logic preserved.
