feat(skills): add skillpm-compatible npm packages for all skills

- Create packages/copilot-cli-guide with SKILL.md as npm package
- Create packages/copilot-setup-audit with SKILL.md as npm package
- Create packages/copilot-skill-builder with SKILL.md, configs (agent + instructions)
- Add root package.json with npm workspaces for monorepo support
- Update README.md with skillpm as recommended install method
- All packages have "agent-skill" keyword, ready for skillpm publish

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs: add missing CLI slash commands to cli-guide skill

Added /clear, /streamer-mode, /exit, /quit, /login, /logout,
/user, and /terminal-setup for complete command coverage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

feat: initial release — CLI-only Copilot customization blueprint

Blueprint for customizing GitHub Copilot CLI with custom agents,
instructions, skills, and MCP server configurations.

No VS Code. No IDE. Just the terminal.

Includes:
- Custom agent: Copilot Customization Builder
- Skills: skill-builder, setup-audit, cli-guide
- Scoped instructions for Markdown files
- AGENTS.md for root-level agent instructions
- Install script for existing projects
- GitHub Actions workflows (release, validate, commit-lint)
- Issue templates, PR template, dependabot config
- Conventional commits enforcement via git hooks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>