docs: reorganize documentation and establish branch governance

[OOXXXXOO]

Summary

• reorganize current documentation under reference, architecture, and operations
• archive historical analyses, plans, and assets under docs/archive
• synchronize CLAUDE.md and repository AGENTS.md
• add staged Feature and Integration GitHub Actions workflows
• document the protected feature/* → develop → main workflow

Verification

• npm ci
• npm run typecheck
• npm test -- --reporter=dot
• npm run build:embedded
• npm pack --dry-run
• cargo test --workspace -- --skip web_fetch_returns_error_for_unreachable_url
• workflow YAML trigger/job assertions
• git diff --check