Add bundled CLI skills

[V3RON]

Summary

• bundle Skillgym skill markdown files inside the published CLI package and expose them via skillgym skills list and skillgym skills get <name>
• add a core skill that acts as the main entrypoint and points agents to focused domain skills for assertions, test cases, workspaces, snapshots, and reporters
• update help and README discovery paths so agents are directed to skillgym skills get core, and cover the new CLI behavior with tests

Closes #22.

Testing

• npm run build
• npx vitest run test/cli.test.ts
• node ./dist/cli.js skills list
• node ./dist/cli.js skills get core

Notes

• npm test currently picks up historical .skillgym-results/**/tests/*.test.* artifacts in this workspace, so I used targeted CLI verification for this change.