fix: clear skill search index when the catalog becomes empty

[AWUQAQ]

Change Summary

After a search populates the skill index, disabling or removing all
skills leaves stale search results. _ensure_indexed() skips indexing
when there are no documents but still updates the cache version.

Reset the retriever when there are no enabled skill documents.

Add regression tests covering:

• Disabling or removing all skills and repeating the search.
• Restoring search after enabling or adding a skill.
• Clearing the child indexes of a hybrid retriever.

Validation

• The three new regression tests fail before the fix.
• All 37 tests in tests/skills/test_skill_features.py pass with the fix.
• All applicable pre-commit hooks pass for both changed files.
• git diff --check passes.

Full-repository checks and remote CI have not been run.
Implementation and regression tests were prepared with AI assistance.

Related issue number

Fixes #974

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check.
• Documentation reflects the changes where applicable

Pre-commit was run on the two changed files; the full-repository
checklist item remains unchecked. No documentation changes are needed
for this internal bug fix.