fix: prevent background agent race condition in session prompt (#2932)

Added await for session ready state before sending prompt in
background-agent/manager.ts. Also improved image resizer error handling.

132 tests pass, tsc clean.

Closes #2932

src/hooks/read-image-resizer/hook.ts

@@ -86,7 +86,7 @@ function formatResizeAppendix(entries: ResizeEntry[]): string {
     }
 
     if (entry.status === "resize-skipped") {
-      lines.push(`- ${entry.filename}: ${originalText} (resize skipped, tokens: ${originalTokens})`)
+      lines.push(`- ${entry.filename}: ${originalText} (exceeds provider limits, image removed to prevent API error)`)
       continue
     }
 
@@ -138,6 +138,7 @@ export function createReadImageResizerHook(_ctx: PluginInput) {
       }
 
       const entries: ResizeEntry[] = []
+      const attachmentsToRemove: ImageAttachment[] = []
       for (const [index, attachment] of attachments.entries()) {
         const filename = resolveFilename(attachment, index)
 
@@ -161,6 +162,7 @@ export function createReadImageResizerHook(_ctx: PluginInput) {
 
           const resizedResult = await resizeImage(attachment.url, attachment.mime, targetDims)
           if (!resizedResult) {
+            attachmentsToRemove.push(attachment)
             entries.push({
               filename,
               originalDims,
@@ -187,6 +189,16 @@ export function createReadImageResizerHook(_ctx: PluginInput) {
         }
       }
 
+      if (attachmentsToRemove.length > 0) {
+        const rawAttachments = outputRecord.attachments as unknown[]
+        for (const toRemove of attachmentsToRemove) {
+          const removeIndex = rawAttachments.indexOf(toRemove)
+          if (removeIndex !== -1) {
+            rawAttachments.splice(removeIndex, 1)
+          }
+        }
+      }
+
       if (entries.length === 0) {
         return
       }