fix(api): handle first-use status and skill search

[370025263]

Summary

Fix first-use behavior when the skill repository or skill index has not been initialized yet.

Changes

• Return an empty skill-search result before creating/probing the embedding client when .skill_index.pkl is missing.
• Let /api/v1/status return 200 with git_branch: null for a non-git skill directory instead of raising a 500.
• Add regression coverage for SDK search, API search, and status on a fresh skill dir.

Test plan

• make test passes
  • Ran make test: 932 passed, 1 failed.
  • Failure: tests/test_e2e_xskill_serve_auto.py::test_canary_flip_promote_and_install_new_version timed out waiting for canary promotion.
  • Reran that single test and it failed with the same canary promotion timeout.
• Added/updated unit tests for the change
  • python3.11 -m pytest tests/test_first_use_status_search.py tests/test_server_no_llm_fallback.py tests/test_server_chat_removed.py tests/test_team_create_app.py -q -> 13 passed.
• make e2e passes (if the change touches ingestion / install / daemon)
• Manually verified the affected user flow

Linked issues

Addresses #46

[370025263]

不知道干啥的可以解释下吗