feat: add agent-driven installation contract

[chasechou007]

Summary

• publish a canonical Agent installation contract and expose the natural-language Codex workflow in both READMEs
• install the alpha CLI as a global user tool or use a checksum-verified standalone fallback without changing host package metadata
• add an isolated global-install smoke that proves Codex initialization, diagnostics, validation, and generated routing, and run it in CI
• add evidence to candidate-0012 while keeping human acceptance and external-adoption review pending

Validation

• npm test (178 passed)
• npm run smoke:agent-bootstrap
• npm run smoke:package
• npm run opendomain -- validate (26 documents; existing stale candidate-0002 warning)
• openspec validate --all --json (11 passed, private nested repository)
• npm run opendomain -- assure openspec/changes/add-agent-driven-installation (not_required; no accepted evidence or Candidate boundaries)

Boundaries

• Homebrew remains deferred
• private OpenSpec artifacts are not included in this public PR
• the npm package gains INSTALL.md on the next prerelease publish