Add sandbox templates and agent runs

[arslnb]

Summary

• add org-scoped sandbox templates with CRUD and batch spawn routes
• add async agent-run orchestration for Codex/Claude Code with secret-backed provider keys and optional control-plane GitHub PR creation
• add TypeScript/Python SDK helpers and docs for templates and agent runs

Tests

• cargo test -p sandboxd
• cargo check -p sandboxd
• npm run build --prefix sdk/typescript
• python3 -m py_compile sdk/python/src/workdir/init.py