Adds a transitive dependency closure capability to task_get so MCP
clients can fetch upstream + downstream blockers in one call instead
of repeatedly walking the graph via task_get.

- TaskService.getDependencyClosure(taskId): BFS over the blocks edge
  table in both directions, dedupes on diamond DAGs, ignores related
  and duplicate relationship types. Returns blockers_transitive,
  blocking_transitive, depth_blockers, depth_blocking.
- task_get include enum extended with 'transitive_deps' (mcp.ts +
  mcp-handlers.ts).
- 5 new unit tests in tests/tasks.test.ts: isolated, 4-level chain,
  diamond DAG, edge-type filter, not-found.
- bench/README.md: neutral framing, no external references; refreshed
  dep-aware-mgmt headline (8.5/10) with the new capability in place.
- bench/visibility/runner.ts: manager prompt updated to mention the
  new include option so the bench actually exercises it.