fix(phase): skip 999.x backlog phases in phase-add numbering (#1950)

Backlog phases use 999.x numbering and should not be counted when
calculating the next sequential phase ID. Without this fix, having
backlog phases causes the next phase to be numbered 1000+.

Co-authored-by: gg <grgbrasil@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/phase.test.cjs

@@ -655,6 +655,43 @@ describe('phase add command', () => {
     const roadmap = fs.readFileSync(path.join(tmpDir, '.planning', 'ROADMAP.md'), 'utf-8');
     assert.ok(roadmap.includes('**Requirements**: TBD'), 'new phase entry should include Requirements TBD');
   });
+
+  test('skips 999.x backlog phases when calculating next phase number', () => {
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'ROADMAP.md'),
+      `# Roadmap v1.0
+
+### Phase 1: Foundation
+**Goal:** Setup
+
+### Phase 2: API
+**Goal:** Build API
+
+### Phase 3: UI
+**Goal:** Build UI
+
+### Phase 999.1: Future Idea A
+**Goal:** Backlog item
+
+### Phase 999.2: Future Idea B
+**Goal:** Backlog item
+
+---
+`
+    );
+
+    const result = runGsdTools('phase add Dashboard', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_number, 4, 'should be phase 4, not 1000');
+    assert.strictEqual(output.slug, 'dashboard');
+
+    assert.ok(
+      fs.existsSync(path.join(tmpDir, '.planning', 'phases', '04-dashboard')),
+      'directory should be 04-dashboard, not 1000-dashboard'
+    );
+  });
 });
 
 // ─────────────────────────────────────────────────────────────────────────────