Fixes#28126. sync_skills() was unconditionally writing bundled skills
into the local <profile_home>/skills/ tree even when the profile's
config.yaml delegated skill resolution to an external directory
via skills.external_dirs. The skill loader then saw two candidates
for the same name (local shadow + external canonical), refused to
resolve on collision, and every worker that auto-loaded such a skill
crashed with 'Unknown skill(s): <name>'.
Changes:
- _build_external_skill_index() indexes skills available in external
dirs (by directory name and frontmatter name)
- sync_skills() skips writing a bundled skill when it finds the same
name in the external index; records the hash in the manifest so
subsequent syncs treat it as already handled
- Self-healing: removes stale local shadows left by prior buggy syncs
(only when origin_hash == bundled_hash == user_hash, i.e. we wrote
it and user didn't touch it)
- New 'shadowed_by_external' key in sync_skills() return dict
3 new tests in TestExternalDirsIndexing (all passing).
All 48 tests in test_skills_sync.py pass.
Closes#28126
db11849c9d