feat: default to Gemini on OpenRouter; drop model-format paragraph

Default OpenRouter runs to google/gemini-3-flash-preview (agent) and
google/gemini-3.1-flash-lite-preview (parsing) when the user doesn't pin a
model. Gated on provider==openrouter, so other providers still defer to their
own engine default (multi-provider intact). Both slugs verified to exist on
OpenRouter. Remove the 'Model format on OpenRouter ...' README paragraph and
update the Inputs table.

Author: brovatten

README.md

@@ -107,8 +107,6 @@ Models are optional. Omit `agent_model` and `parsing_model` to use the engine's
           parsing_model: google/gemini-3-flash-preview      # optional
 ```
 
-Model format on OpenRouter is a bare slug (e.g. `anthropic/claude-sonnet-4`): exactly one `/`, with no `openrouter/` prefix (that is the LiteLLM form, which the action rejects early). Other providers use their own native model ids.
-
 ## Bring your own LLM provider
 
 OpenRouter is the default, but you can use any provider the engine supports. Set `llm_provider` and pass that provider's key:
@@ -159,8 +157,8 @@ The command needs the `issue_comment` trigger and runs from your default branch
 | `render_depth` | `1` | Display depth for the PR diagram. Keep `1` for a clean top-level view. |
 | `diagram_direction` | `LR` | Mermaid direction: `LR`, `TD`, `TB`, `RL`, or `BT`. |
 | `changed_only` | `false` | Render only changed components and incident edges. |
-| `agent_model` | engine default | Analysis model. Bare OpenRouter slug (e.g. `anthropic/claude-sonnet-4`); empty uses the engine's per-provider default. |
-| `parsing_model` | engine default | Parsing model. Bare OpenRouter slug; empty uses the engine's per-provider default. |
+| `agent_model` | `google/gemini-3-flash-preview` | Analysis model. OpenRouter default shown; other providers use their own engine default. |
+| `parsing_model` | `google/gemini-3.1-flash-lite-preview` | Parsing model. OpenRouter default shown; other providers use their own engine default. |
 | `comment_header` | `Architecture review` | Heading for the PR comment. |
 | `trigger_command` | `/codeboarding` | Slash command for trusted on-demand runs. |
 | `cta_base_url` | empty | Optional click-proxy base URL for editor and extension links. |

action.yml

@@ -27,11 +27,11 @@ inputs:
     required: false
     default: '1'
   agent_model:
-    description: 'Analysis model (AGENT_MODEL env var). A bare OpenRouter slug, e.g. anthropic/claude-sonnet-4. Empty (default) uses the engine''s own per-provider default.'
+    description: 'Analysis model (AGENT_MODEL env var). A bare OpenRouter slug. Defaults to google/gemini-3-flash-preview on OpenRouter; for other providers, empty uses the engine''s per-provider default.'
     required: false
     default: ''
   parsing_model:
-    description: 'Parsing model (PARSING_MODEL env var). A bare OpenRouter slug. Empty (default) uses the engine''s own per-provider default.'
+    description: 'Parsing model (PARSING_MODEL env var). A bare OpenRouter slug. Defaults to google/gemini-3.1-flash-lite-preview on OpenRouter; for other providers, empty uses the engine''s per-provider default.'
     required: false
     default: ''
   comment_header:
@@ -282,6 +282,10 @@ runs:
         echo "Provider: $PROVIDER -> $PROVIDER_ENV; key length: ${#KEY}"
 
         if [ "$PROVIDER" = "openrouter" ]; then
+          # Default models on OpenRouter when the user didn't pin one (cheap Gemini).
+          # Other providers fall through to the engine's own per-provider default.
+          AGENT_MODEL="${AGENT_MODEL:-google/gemini-3-flash-preview}"
+          PARSING_MODEL="${PARSING_MODEL:-google/gemini-3.1-flash-lite-preview}"
           # OpenRouter-only checks. The litellm 'openrouter/...' model prefix 400s
           # the engine's native OpenRouter call; other providers use native ids.
           for M in "$AGENT_MODEL" "$PARSING_MODEL"; do