[Feature Request] Support project-scoped local skills (`.claude/skills.local/` or `local: true` frontmatter)

Bug Description
Skills currently support Project (.claude/skills/), User (~/.claude/skills/), Enterprise, and Plugin scopes — but no documented equivalent to settings.local.json or CLAUDE.local.md for personal, project-scoped skills.

Use case: I want skills that are specific to one project but NOT shared with my team via version control. Today I have to either:

1. Gitignore .claude/skills/ wholesale (breaks if the team commits shared skills there)
2. Use .git/info/exclude with a naming convention like local-* (works but undocumented)
3. Dump them in ~/.claude/skills/ where they load in every project (wrong scope)

Proposed: support .claude/skills.local/ (auto-gitignored like settings.local.json) or a local: true field in SKILL.md frontmatter that opts a skill out of being shared.