feat: add skill installation via hook install

[jung-thomas]

Summary

• Adds skills.yaml as a new content type in packs, parallel to hook.yaml and mcp.yaml
• sap-devs hook install tutorial copies the /tutorial skill to ~/.claude/skills/tutorial/SKILL.md
• sap-devs hook install (no args) installs all hooks AND all skills for the active profile
• hook list, hook status, and hook uninstall extended to handle skills alongside hooks
• Ships the tutorial instructor skill as the first distributable skill in the base pack

New files

• internal/content/skill.go — FlattenSkills(), FindSkillDef() helpers
• internal/adapter/skill_wire.go — WriteSkillFile(), RemoveSkillFile(), SkillFileInstalled()
• content/packs/base/skills.yaml — tutorial skill definition
• content/packs/base/skills/tutorial/SKILL.md — tutorial skill content
• content/schemas/skills.schema.json — JSON Schema for validation

Modified files

• internal/content/pack.go — SkillDef struct, Skills field, loading in LoadPack()
• internal/content/merge.go — mergeSkills() for additive layer support
• internal/adapter/adapter.go — SkillConfig struct and field on Adapter
• content/adapters/claude-code.yaml — skill_config: path: "~/.claude/skills"
• cmd/hook.go — install/uninstall/list/status extended for skills
• internal/schema/schema.go — registered skills.yaml schema
• internal/i18n/catalogs/en.json — 6 skill i18n strings

Test plan

• go build ./... && go vet ./... passes clean
• sap-devs hook list --all shows tutorial skill in a separate skills table
• sap-devs hook install tutorial --dry-run prints expected write path
• sap-devs hook install tutorial creates ~/.claude/skills/tutorial/SKILL.md
• sap-devs hook status shows tutorial skill as installed
• sap-devs hook uninstall tutorial removes the skill file
• sap-devs hook install (no args) installs hooks AND the tutorial skill
• /tutorial in Claude Code triggers the skill correctly