feat(manager): add interactive command center with dependency-aware recommendations

Add /gsd:manager — a single-terminal dashboard for managing milestones.
Shows all phases with visual status indicators (D/P/E columns), computes
recommended next actions, and dispatches discuss inline while plan/execute
run as background agents.

Key behaviors:
- Recommendation engine prioritizes execute > plan > discuss
- Filters parallel execute/plan when phases share dependency chains
- Independent phases (no direct or transitive dep relationship) CAN
  run in parallel — dependent phases are serialized
- Dashboard shows compact Deps column for at-a-glance dependency view
- Sliding window limits discuss to one phase at a time
- Activity detection via file mtime (5-min window) for is_active flag

New files:
- commands/gsd/manager.md — skill definition
- get-shit-done/workflows/manager.md — full workflow spec
- get-shit-done/bin/lib/init.cjs — cmdInitManager() with phase parsing,
  dependency graph traversal, and recommendation filtering
- get-shit-done/bin/gsd-tools.cjs — route 'init manager' to new command
- tests/init-manager.test.cjs — 16 tests covering status detection,
  deps, sliding window, recommendations, and edge cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package-lock.json

@@ -16,7 +16,7 @@
         "esbuild": "^0.24.0"
       },
       "engines": {
-        "node": ">=16.7.0"
+        "node": ">=20.0.0"
       }
     },
     "node_modules/@bcoe/v8-coverage": {