Add tests: cover docstring_sync.py at 17% coverage

[neuromechanist]

Summary

src/knowledge/docstring_sync.py has only 17% test coverage (46-136, 156-195, 213-222 are uncovered). This is a critical module for extracting Python docstrings from external repos — if it fails silently, the knowledge base will have gaps.

Current Coverage

src/knowledge/docstring_sync.py    93 lines,   17% covered

Uncovered code:

• Core sync logic (lines 46-136): repo cloning, docstring extraction
• Multi-language handling (lines 156-195)
• Error handling path (lines 213-222)

Acceptance Criteria

• Coverage for docstring_sync.py reaches >=70%
• Tests cover: valid repo clone, empty repo, invalid repo URL
• Multi-language branch (python) tested with real repo
• Error paths (network failure, git clone failure) covered
• Follow NO-MOCKS policy — use real repos or skip