`hermes update` keeps (won't overwrite) bundled skills the user edited
locally, but only printed a count — "~ N user-modified (kept)" — with no way
to learn which skills, or see what changed. Reverting already existed
(`hermes skills reset <name> [--restore]`); discovery and inspection did not.
Add two CLI commands (zero model-tool footprint), reusing the manifest
origin-hash that sync already maintains:
- `hermes skills list-modified [--json]` — list the bundled skills whose
on-disk copy diverges from the last-synced origin hash (the exact test the
sync loop uses to decide what to skip).
- `hermes skills diff <name>` — unified diff between the user's copy and the
current bundled (stock) version, so the user can confirm what changed
before reverting.
Both are mirrored as `/skills list-modified` and `/skills diff`. The
`hermes update` notice now points at `hermes skills list-modified`. Core
helpers `list_user_modified_bundled_skills()` and `diff_bundled_skill()` live
in tools/skills_sync.py alongside the existing reset logic.
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