function folder/package support (not only single-file functions)

## Summary
Support function-as-folder layout instead of single-file functions.

## Motivation
Real functions often need multiple files (helpers, schema, assets, dependency manifests). Current single-file model limits maintainability.

## Proposal
- Extend function model to allow directory roots, e.g. `functions/ts/my_fn/`.
- Runtime loader resolves entrypoint (`index.ts`, `main.py`, `mod.rs`, etc.) inside folder.
- Hook sync copies full function directories, not just top-level files.
- Keep single-file mode backward compatible.

## Acceptance criteria
- Portal create/update APIs accept folder-based functions.
- Runtimes (at least TS/Python/Lua) load folder entrypoints.
- Existing single-file functions continue to run unchanged.

Start by tracing the existing single-file function model, portal create/update APIs, runtime loader, and hook sync behavior. Define how folder roots and entrypoints work for TypeScript, Python, and Lua while preserving single-file behavior; done means the stated APIs, runtimes, and backward-compatibility criteria pass.

Written by the indexing model from the issue text.