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>

tests/copilot-install.test.cjs

@@ -625,7 +625,7 @@ describe('copyCommandsAsCopilotSkills', () => {
       // Count gsd-* directories — should be 31
       const dirs = fs.readdirSync(tempDir, { withFileTypes: true })
         .filter(e => e.isDirectory() && e.name.startsWith('gsd-'));
-      assert.strictEqual(dirs.length, 50, `expected 50 skill folders, got ${dirs.length}`);
+      assert.strictEqual(dirs.length, 51, `expected 51 skill folders, got ${dirs.length}`);
     } finally {
       fs.rmSync(tempDir, { recursive: true });
     }
@@ -1119,7 +1119,7 @@ const { execFileSync } = require('child_process');
 const crypto = require('crypto');
 
 const INSTALL_PATH = path.join(__dirname, '..', 'bin', 'install.js');
-const EXPECTED_SKILLS = 50;
+const EXPECTED_SKILLS = 51;
 const EXPECTED_AGENTS = 16;
 
 function runCopilotInstall(cwd) {