feat(plugins): pass attachment metadata to before_model_resolve hook

Add optional `attachments` field to `PluginHookBeforeModelResolveEvent`
so plugins can route models based on file type (images, documents, etc.).

This enables file-aware model routing — e.g. routing image-heavy
messages to vision-optimized models like Gemini Flash instead of
defaulting to Sonnet, significantly reducing cost without quality loss.

The change is fully backwards-compatible: `attachments` is optional
and existing plugins continue to work without modification.

src/agents/pi-embedded-runner/run.ts

@@ -300,8 +300,14 @@ export async function runEmbeddedPiAgent(
         channelId: params.messageChannel ?? params.messageProvider ?? undefined,
       };
 
+      const attachments = (params.images ?? []).map((img) => ({
+        kind: "image" as const,
+        mimeType: img.media_type,
+      }));
+
       const hookSelection = await resolveHookModelSelection({
         prompt: params.prompt,
+        attachments: attachments.length > 0 ? attachments : undefined,
         provider,
         modelId,
         hookRunner,

src/agents/pi-embedded-runner/run/setup.ts

@@ -28,7 +28,7 @@ type HookContext = {
 type HookRunnerLike = {
   hasHooks(hookName: string): boolean;
   runBeforeModelResolve(
-    input: { prompt: string },
+    input: { prompt: string; attachments?: { kind: string; mimeType?: string }[] },
     context: HookContext,
   ): Promise<{ providerOverride?: string; modelOverride?: string } | undefined>;
   runBeforeAgentStart(
@@ -39,6 +39,7 @@ type HookRunnerLike = {
 
 export async function resolveHookModelSelection(params: {
   prompt: string;
+  attachments?: { kind: string; mimeType?: string }[];
   provider: string;
   modelId: string;
   hookRunner?: HookRunnerLike | null;
@@ -58,7 +59,7 @@ export async function resolveHookModelSelection(params: {
   if (hookRunner?.hasHooks("before_model_resolve")) {
     try {
       modelResolveOverride = await hookRunner.runBeforeModelResolve(
-        { prompt: params.prompt },
+        { prompt: params.prompt, attachments: params.attachments },
         params.hookContext,
       );
     } catch (hookErr) {

src/plugins/hook-before-agent-start.types.ts

@@ -1,7 +1,14 @@
 // before_model_resolve hook
+export type PluginHookBeforeModelResolveAttachment = {
+  kind: "image" | "video" | "audio" | "document" | "other";
+  mimeType?: string;
+};
+
 export type PluginHookBeforeModelResolveEvent = {
   /** User prompt for this run. No session messages are available yet in this phase. */
   prompt: string;
+  /** Attachment metadata for file-aware model routing. */
+  attachments?: PluginHookBeforeModelResolveAttachment[];
 };
 
 export type PluginHookBeforeModelResolveResult = {