fix: Persist and expose task dependencies from detail output

Detail agents define task dependencies in YAML frontmatter but they were
silently dropped — never written to the task_dependencies table. This
caused all tasks to dispatch in parallel regardless of intended ordering,
and the frontend showed no dependency information.

- Add fileIdToDbId mapping and second-pass dependency creation in
  output-handler.ts (mirrors existing phase dependency pattern)
- Add task_dependency to changeset entry entityType enum
- Add listPhaseTaskDependencies tRPC procedure for batch querying
- Wire blockedBy in PhaseDetailPanel and PhaseWithTasks from real data
- Clarify dependency semantics in detail prompt

docs/server-api.md

@@ -79,6 +79,7 @@ Each procedure uses `require*Repository(ctx)` helpers that throw `TRPCError(INTE
 | listPhaseTasks | query | All tasks for phase |
 | listPendingApprovals | query | Tasks with status=pending_approval |
 | deleteTask | mutation | Delete a task by ID |
+| listPhaseTaskDependencies | query | All task dependency edges for tasks in a phase |
 | approveTask | mutation | Approve and complete task |
 
 ### Initiatives