refactor: adopt planningPaths() across 4 modules — eliminate 34 inline path constructions

Consolidates repeated path.join(cwd, '.planning', ...) patterns into
calls to the shared planningPaths() helper from core.cjs.

Modules updated:
- state.cjs: 16 → planningPaths() (2 remain for non-standard paths)
- commands.cjs: 8 → planningPaths() (4 remain for todos dir)
- milestone.cjs: 5 → planningPaths() (3 remain for archive/milestones)
- roadmap.cjs: 4 → planningPaths()

Benefits:
- Single source of truth for .planning/ directory structure
- Easier to change planning dir location in the future
- Consistent path construction across the codebase
- No behavioral changes — pure refactor

797/797 tests pass.

tests/cursor-conversion.test.cjs

@@ -32,7 +32,7 @@ Test body
     const nameMatch = result.match(/^name:\s*(.+)$/m);
 
     assert.ok(nameMatch, 'frontmatter contains name field');
-    assert.equal(nameMatch[1], 'gsd-quick', 'skill name is plain scalar');
+    assert.strictEqual(nameMatch[1], 'gsd-quick', 'skill name is plain scalar');
     assert.ok(!result.includes('name: "gsd-quick"'), 'quoted skill name is not emitted');
   });
 
@@ -75,7 +75,7 @@ Planner body
     const nameMatch = result.match(/^name:\s*(.+)$/m);
 
     assert.ok(nameMatch, 'frontmatter contains name field');
-    assert.equal(nameMatch[1], 'gsd-planner', 'agent name is plain scalar');
+    assert.strictEqual(nameMatch[1], 'gsd-planner', 'agent name is plain scalar');
     assert.ok(!result.includes('name: "gsd-planner"'), 'quoted agent name is not emitted');
   });
 });