docs: use vars (not secrets) for model overrides; state the slug format rule

A model name is non-sensitive config, so steer overrides to a repo variable
(vars.AGENT_MODEL) and keep secrets. for the key. State the format rule once:
bare OpenRouter slug, exactly one '/', no 'openrouter/' prefix.

Author: brovatten

README.md

@@ -78,15 +78,17 @@ OPENROUTER_API_KEY = sk-or-...
 
 That's the only required setup — it's passed via `llm_api_key` above. (For local runs with `scripts/run_local.sh`, export `OPENROUTER_API_KEY` as an env var instead.)
 
-**Models are optional.** Omit `agent_model` / `parsing_model` to use the engine's default for your provider, or pin them to a **bare OpenRouter slug** (no `openrouter/` prefix) — inline or from a secret:
+**Models are optional.** Omit `agent_model` / `parsing_model` to use the engine's default for your provider, or pin them — inline or from a repository **variable** (a model name isn't a secret, so use `vars.`, not `secrets.`):
 
 ```yaml
         with:
-          llm_api_key:   ${{ secrets.OPENROUTER_API_KEY }}
-          agent_model:   anthropic/claude-sonnet-4      # optional; or ${{ secrets.AGENT_MODEL }}
-          parsing_model: google/gemini-3-flash-preview  # optional
+          llm_api_key:   ${{ secrets.OPENROUTER_API_KEY }}  # secret
+          agent_model:   anthropic/claude-sonnet-4          # optional; or ${{ vars.AGENT_MODEL }}
+          parsing_model: google/gemini-3-flash-preview      # optional
 ```
 
+**Model format:** a bare OpenRouter slug (e.g. `anthropic/claude-sonnet-4`) — exactly one `/`, **no `openrouter/` prefix** (that's the LiteLLM form; the action rejects it early).
+
 ## When it runs
 
 - **PR opened / reopened / marked ready** — generated once (per the `on:` triggers above). It does **not** re-run on every push, so you never spend an LLM job per commit; the comment reflects that point until refreshed.