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

apps/server/trpc/routers/task.ts

@@ -151,6 +151,19 @@ export function taskProcedures(publicProcedure: ProcedureBuilder) {
         return { success: true };
       }),
 
+    listPhaseTaskDependencies: publicProcedure
+      .input(z.object({ phaseId: z.string().min(1) }))
+      .query(async ({ ctx, input }) => {
+        const taskRepo = requireTaskRepository(ctx);
+        const tasks = await taskRepo.findByPhaseId(input.phaseId);
+        const edges: Array<{ taskId: string; dependsOn: string[] }> = [];
+        for (const t of tasks) {
+          const deps = await taskRepo.getDependencies(t.id);
+          if (deps.length > 0) edges.push({ taskId: t.id, dependsOn: deps });
+        }
+        return edges;
+      }),
+
     approveTask: publicProcedure
       .input(z.object({ taskId: z.string().min(1) }))
       .mutation(async ({ ctx, input }) => {