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/agent.md

@@ -9,7 +9,7 @@
 | `types.ts` | Core types: `AgentInfo`, `AgentManager` interface, `SpawnOptions`, `StreamEvent` |
 | `manager.ts` | `MultiProviderAgentManager` — main orchestrator class |
 | `process-manager.ts` | `AgentProcessManager` — worktree creation, command building, detached spawn |
-| `output-handler.ts` | `OutputHandler` — JSONL stream parsing, completion detection, proposal creation, task dedup |
+| `output-handler.ts` | `OutputHandler` — JSONL stream parsing, completion detection, proposal creation, task dedup, task dependency persistence |
 | `file-tailer.ts` | `FileTailer` — watches output files, fires parser + raw content callbacks |
 | `file-io.ts` | Input/output file I/O: frontmatter writing, signal.json reading, tiptap conversion |
 | `markdown-to-tiptap.ts` | Markdown to Tiptap JSON conversion using MarkdownManager |