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/image-resizer.ts

@@ -1,6 +1,7 @@
 import type { ImageDimensions, ResizeResult } from "./types"
 import { extractBase64Data } from "../../tools/look-at/mime-type-inference"
 import { log } from "../../shared"
+import { resizeImageFallback } from "./png-fallback-resizer"
 
 const ANTHROPIC_MAX_LONG_EDGE = 1568
 const ANTHROPIC_MAX_FILE_SIZE = 5 * 1024 * 1024
@@ -114,14 +115,14 @@ export async function resizeImage(
     const sharpModuleName = "sharp"
     const sharpModule = await import(sharpModuleName).catch(() => null)
     if (!sharpModule) {
-      log("[read-image-resizer] sharp unavailable, skipping resize")
-      return null
+      log("[read-image-resizer] sharp unavailable, attempting pure-JS fallback")
+      return resizeImageFallback(base64DataUrl, mimeType, target)
     }
 
     const sharpFactory = resolveSharpFactory(sharpModule)
     if (!sharpFactory) {
-      log("[read-image-resizer] sharp import has unexpected shape")
-      return null
+      log("[read-image-resizer] sharp import has unexpected shape, attempting pure-JS fallback")
+      return resizeImageFallback(base64DataUrl, mimeType, target)
     }
 
     const rawBase64 = extractBase64Data(base64DataUrl)