refactor(skills): split package into focused files

[dgageot]

Summary

Internal refactor of pkg/skills to make the package easier to read and maintain. No public API or behavior change.

Why

pkg/skills/skills.go had grown to 402 lines doing five different jobs (types, public API, local discovery, frontmatter parsing, helpers). The precedence rules for local skill loading were buried in nested for-loops. A few one-line wrappers (loadRemoteSkills → loadRemoteSkillsWithCache, defaultCache()) added indirection without value.

What changed

Three small commits:

1. refactor(skills): inline trivial wrappers and use maps.Values

   • Merge loadRemoteSkills wrapper + defaultCache() helper.
   • Merge loadSkillsRecursive + walkSkillsRecursive.
   • Use slices.Collect(maps.Values(...)) for map → slice conversions.
   • Drop redundant os.Stat in loadSkillsFromDir.

2. refactor(skills): split package into focused files

   • skills.go (402 → 82 lines): public API only — Skill type + Load.
   • local.go (new): filesystem discovery, with a declarative localSearchPaths() table that makes the precedence rules (codex → claude → agents → project) easy to read at a glance. findGitRoot+projectSearchDirs share a single walk-up.
   • frontmatter.go (new): YAML-ish frontmatter parsing, split into parseTopLevelLine and parseIndentedLine.
   • Load now sorts its result by Name for deterministic output.
   • Load avoids the double map → slice → map round-trip via loadLocalSkillsInto.
   • Skill.IsFork uses a value receiver for consistency.
   • isHTTPSource extracted from Load.

3. fix(skills): tighten review nits from refactor

   • Load: add FilePath tiebreaker to the sort. slices.SortedFunc uses pdqsort which is unstable, so two skills sharing a Name (e.g. local + remote both exposing foo) would have non-deterministic relative order. FilePath is unique → fully deterministic output.
   • Clarify symlink behavior in loadSkillsFlat (skips symlinks explicitly) and loadSkillsRecursive (filepath.WalkDir does not follow symlinks; the visited map is defensive against bind mounts and symlinked roots, not symlinks inside the tree).

File sizes (before → after)

File	Before	After
skills.go	402	82
local.go	—	232
frontmatter.go	—	122
remote.go	153	136

Validation

• mise lint — clean (golangci-lint + custom cop + go mod tidy --diff)
• mise test — all packages pass
• Public API unchanged: Skill, Skill.IsFork, Load, ExpandCommands keep the same signatures