Sync (per-folder AGENTS.md)

samuel sync walks the project tree and writes per-folder AGENTS.md files describing each directory's purpose, primary languages, key files, and whether tests live there. The framework writes AGENTS.md exclusively — vendor-specific context files are produced by translator plugins, not the core (v2 invariant per RFD 0009).

Behavior

• Autogen marker. Generated files start with the marker <!-- Auto-generated by Samuel. Files without the marker are considered user-customized and skipped unless --force.
• Skip set. Hidden directories, node_modules/, vendor/, language build outputs, and IDE caches are skipped. Configurable via samuel.toml [sync.skip].
• Language map. Extension → language (configurable via [sync.languages]).
• Folder map. Folder name → purpose (configurable via