fix: keep issue metadata only in ~/.codex/AGENTS.md

[skulidropek]

Summary

• stop writing docker-git issue-managed blocks into project AGENTS.md
• keep workspace context only in ${CODEX_HOME}/AGENTS.md
• add guards to block committing/pushing managed issue block changes in repo AGENTS.md
• remove stale issue-managed block from current AGENTS.md

Testing

• pnpm --filter ./packages/docker-git test -- tests/core/templates.test.ts
• pnpm --filter ./packages/lib test
• pnpm --filter ./packages/docker-git typecheck

Closes #67