Add project-scope skill installation

[michael-webster]

Summary

• Introduces a Scope enum (ScopeUser, ScopeProject) to the internal/skills package — all callers now pass a scope explicitly
• User scope (default): existing behavior unchanged — installs into ~/.claude/skills and ~/.agents/skills, skipping agents whose config dirs don't exist
• Project scope: installs into .claude/skills and .agents/skills relative to the current working directory, creating directories as needed (no pre-existing agent config required)
• Adds --scope user|project flag to both skill install and skill list (default: user)

Reproduction commands

# User scope (existing behavior, unchanged)
chunk skill install
chunk skill install --scope user

# Project scope — installs into ./.claude/skills and ./.agents/skills
chunk skill install --scope project

# List with project scope (agents always shown as available)
chunk skill list --scope project

# Invalid scope returns a clear error
chunk skill install --scope global
# → error: invalid scope: must be 'user' or 'project'

Test plan

• All existing acceptance tests pass (user scope unchanged)
• TestSkillsInstallProjectScope — installs to project dir, leaves user home untouched
• TestSkillsInstallProjectScopeUpToDate — idempotent on second run
• TestSkillsInstallProjectScopeNotIsolatedFromUserScope — user and project installs coexist
• TestSkillsListProjectScope — list shows missing before install, current after
• TestSkillsInstallInvalidScope — unknown scope returns non-zero exit + clear error
• Unit tests for ScopeProject in internal/skills/skills_test.go

🤖 Generated with Claude Code