Add install.exclude frontmatter to keep companion files out of platform copies (#103)

[devrimcavusoglu]

Summary

Fixes #103. Stacked on #106 (base branch fix/frontmatter-passthrough) — it needs #100's modeled-key machinery so the install: block survives registry writes. Once #106 merges, retarget this PR to main (GitHub will do it automatically on branch deletion).

• New top-level frontmatter block, author-owned:

  install:
    exclude: [eval, fixtures/*, "*.draft.md"]

  Modeled as Skill.Install.Exclude (InstallConfig), omitted from output when empty.
• skill.MatchExclude(patterns, rel) — stdlib path.Match against the slash-relative path and each leading directory, so a bare dir name excludes its subtree, fixtures/* its children, *.draft.md top-level files by suffix. No ** (documented, no new deps). SKILL.md can never be excluded.
• Platform.Install(skillDir, name, scope, opts InstallOptions) — copyDir takes a skip func and prunes matched directories with filepath.SkipDir. skill_install.go already had the parsed skill from reg.Get (it was discarding it); it now passes s.Install.Exclude. Registry copies (create --from-template, import) are untouched — the registry keeps everything.
• skill validate: errors for empty / absolute / .. / malformed-glob / matches-SKILL.md patterns; warnings (in ValidateFolder) when a pattern matches no file, or when a body-referenced file would be excluded (exists in registry, missing from every install).
• Docs: skill-format "Install-time exclusions" section + table row, validation reference, platform-adapters how-it-works, commands install, AGENTS.md format block + design decision 3 wording; CHANGELOG [Unreleased] › Added.

Issue DoD

• Named directories/globs under a skill can be excluded from skill install while staying in the registry
• skill diff accounts for intentionally-excluded paths — nothing to change: diff compares manifests (frontmatter + body), never file trees, so excluded files cannot surface as drift. TestSkillInstall_HonorsInstallExclude asserts diff reports identical after an excluding install. Documented in the new section.

Test plan

• TestMatchExclude (17 cases), TestValidateExcludePattern, TestExcludedFiles, TestValidate_InstallExclude, TestValidateFolder_InstallExcludeWarnings, TestAdapter_Install_Exclude, TestCopyDir_SkipFuncPrunesDirectories, TestSkillInstall_HonorsInstallExclude (issue scenario end-to-end incl. round-trip of the block), TestSkillValidate_InstallExcludeErrors
• go test ./..., make lint green
• Manual: template with install.exclude: [eval] + eval/s1.md → registry has eval/s1.md, .claude/skills/demo.skill/ has only SKILL.md; bad patterns → validate exit 2

🤖 Generated with Claude Code

[devrimcavusoglu]

Review (adversarial pass against #103)

DoD 1 holds across every registry write path (create, from-template, import keep everything; install trims). DoD 2 — file trees never show as drift (no skern surface walks installed trees), but the PR had introduced a regression in the same area, now fixed (8a78dad):

Sev	Finding	Resolution
Medium	skill diff went blind to install.exclude changes — install became a modeled key so it left Extra, and compareSkills never looked at it	install.exclude diffed as a field; install.* extras diffed too (TestSkillDiff_InstallExcludeAndExtras)
Medium	Unmodeled keys under install: (install.mode, …) were dropped — undoing #100's guarantee for that subtree	InstallConfig.Extra inline map + collision guard; exclude: eval (bare string) accepted as a one-element list
Low/Med	skill install silently ignored an invalid pattern (path.Match just returns false → the file the author meant to exclude gets installed)	Install refuses the skill with a per-skill error pointing at skill validate (TestSkillInstall_RefusesInvalidExcludePattern)
Low	** silently degraded to * (works on a shallow fixture tree, fails deeper)	** is a validation error with an explanatory message
Low	Rejecting every pattern that could match SKILL.md (*, *.md) contradicted "SKILL.md is never excluded" — validate exited 2 for a config install honored correctly	Only the literal SKILL.md is an error; * = "everything except SKILL.md"
Low	Backslash→slash normalization broke path.Match's \ escape	Backslashes left alone; documented
Low	Docs cited a nonexistent skill show --files; compatibility note for non-mapping install: missing	Fixed; CHANGELOG carries the compatibility note
Nit	Multi-entry error messages lacked an index; json:"…,omitempty" on a struct is a no-op	install.exclude[N]; omitzero

Stacked on #108 (base feature/install-tag-filter) and rebased onto the updated #106; CI runs once the base becomes main. Full suite + lint green locally at the stack tip.