fix: review fixes — path traversal guard, trust_style consistency, edge cases

Address code review findings:

Security (Medium):
- Path traversal guard in OptionalSkillSource.fetch() — resolve() and
  validate that the path stays within optional-skills/ before reading

Bug fixes (Medium):
- Add 'builtin' to trust_style dicts in do_inspect() and
  _resolve_short_name() — official skills now show bright_cyan 'official'
  label consistently across all display functions (5/5 dicts fixed)

Edge cases (Low):
- Clamp page_size to [1, 100] in do_browse() to prevent ZeroDivisionError
- Update SkillMeta.source docstring to include 'official'
- Add browse command to optional-skills/DESCRIPTION.md

optional-skills/DESCRIPTION.md

@@ -6,8 +6,10 @@ These skills ship with the hermes-agent repository but are not copied to
 `~/.hermes/skills/` during setup. They are discoverable via the Skills Hub:
 
 ```bash
-hermes skills search <query>     # finds optional skills labeled "official"
-hermes skills install <identifier>  # copies to ~/.hermes/skills/ and activates
+hermes skills browse               # browse all skills, official shown first
+hermes skills browse --source official  # browse only official optional skills
+hermes skills search <query>       # finds optional skills labeled "official"
+hermes skills install <identifier> # copies to ~/.hermes/skills/ and activates
 ```
 
 ## Why optional?