fix: embeddings from config + remove IMAGE_ROOT validation #46

Two fixes:

1. embeddings.ts: reads model/key/base_url/dimensions from config instead of process.env. env vars are never set by the config system (TOML+secrets parsing doesn't export to process.env), so embeddings were silently disabled.

2. tools.ts: removed IMAGE_ROOT path validation from readImageForModel. nano owns her entire container - there is no multi-tenant threat model. the validation only caused bugs (computer use screenshots saved to /tmp could never pass the IMAGE_ROOT check). readImageForModel now just normalizes the path and reads it.