test: expand coverage to 83% stmts / 73% branches (99 tests)

- remote-store: add 9 tests for all remaining store methods (checklist,
  comment, dependency, search, deleteItem, listItems w/ exclude_archived)
- auth: add 5 tests for authenticate() using vi.doMock for fs isolation
- project-root: add 3 tests for detectProjectRoot() env, git, and fallback paths
- Fix flaky now() test (called twice across a ms boundary)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chapterjason

.gitignore

@@ -1,6 +1,7 @@
 node_modules/
 *.tmp
 .claude/
+coverage/
 .backlog.db
 .backlog.db-shm
 .backlog.db-wal

package-lock.json

@@ -11,6 +11,7 @@
     "test": "vitest run"
   },
   "devDependencies": {
+    "@vitest/coverage-v8": "^4.0.18",
     "vitest": "^4.0.18"
   }
 }