Change engine/.env to be committed and have .env.local override (#6150)

# Description of Changes
We keep adding stuff to `engine/config/.env.example` and have to
manually update `.env` because of it, which is really clunky, especially
when working on multiple worktrees at once. This PR changes it so that
we just have a committed `.env` file and have an `.env.local` override
to put the actual private keys into, which should make it a bit easier
to manage.

> [!warning]
>
> After this goes in, be very careful for a little while not to
accidentally commit any keys that you've got inside your `.env` file!

engine/src/stirling/rag/README.md

@@ -37,7 +37,9 @@ multi = RagCapability(runtime.rag_service, collections=["company-docs", "product
 everything = RagCapability(runtime.rag_service)
 ```
 
-## Config (.env)
+## Config
+
+Non-secret defaults live in the committed `engine/.env`:
 
 ```
 STIRLING_RAG_BACKEND=sqlite              # or "pgvector"
@@ -47,6 +49,11 @@ STIRLING_RAG_PGVECTOR_DSN=               # used when backend=pgvector
 STIRLING_RAG_CHUNK_SIZE=512
 STIRLING_RAG_CHUNK_OVERLAP=64
 STIRLING_RAG_TOP_K=5
+```
+
+Provider credentials (and any local overrides) go in the uncommitted `engine/.env.local`:
+
+```
 VOYAGE_API_KEY=your-key
 ```