docs: add agent-first public guidance

[chasechou007]

Summary

• make natural-language Codex installation and follow-up prompts the primary README path
• add complete English and Simplified Chinese Usage Guides for greenfield, Brownfield, Candidate review, grounding, integrations, and recovery
• verify public links, CLI consistency, npm package contents, and private process-record exclusion

Validation

• npm test (181 passed)
• node --test tests/public-docs.test.mjs (3 passed)
• npm run smoke:agent-bootstrap
• npm run smoke:package
• npm pack --dry-run --json --cache /tmp/opendomain-npm-cache (64 entries)
• npm run opendomain -- validate (26 documents, one pre-existing stale Candidate warning)
• openspec validate --all --json (13/13 passed in the private process repository)

This change does not modify runtime behavior, schemas, accepted domain semantics, npm versions, tags, or releases. Private OpenSpec and development records are not included.