feat: add image hash-based OCR caching with 100MB size limit

- Implement SHA-256 image hashing to cache OCR results and avoid re-processing identical images
- Add 100MB file size limit for image uploads with user-friendly error messages
- Clear ghost suggestions when uploading new images to prevent interference
- Optimize size limit calculation in copilot plugin to include OCR context
- Remove debug logging from production code
- Add image processing optimization plan document

BREAKING CHANGE: Image upload size limit is now enforced at 100MB (previously unlimited)

src/plugins/copilotPlugin.ts

@@ -9,7 +9,6 @@ import { getOcrCache, OCR_SIZE_LIMIT, extractTextFromOCR } from '../utils/ocrCac
 const COPILOT_PLUGIN_KEY = new PluginKey('milkdown-copilot')
 const DEBOUNCE_MS = 1000
 const SIZE_LIMIT = OCR_SIZE_LIMIT
-const DEBUG = true
 const IMAGE_NODE_TYPES = new Set(['image', 'image-block', 'imageBlock'])
 
 interface CopilotState {
@@ -334,13 +333,8 @@ function scheduleFetch(view: EditorView, runtime: CopilotRuntime, pos: number) {
 
   const doc = view.state.doc
   const schema = view.state.schema
-  const overLimit = doc.content.size > SIZE_LIMIT
-
-  if (overLimit) {
-    setCopilotEnabled(view, false)
-    return
-  }
-
+  const baseSize = doc.content.size
+  
   const serializer = runtime.ctx.get(serializerCtx)
   let prefixMarkdown = ''
   let suffixMarkdown = ''
@@ -362,12 +356,15 @@ function scheduleFetch(view: EditorView, runtime: CopilotRuntime, pos: number) {
   }
 
   const requestPrefix = `${prefixMarkdown}${buildOcrContextForRequest(doc, pos)}`
+  const totalTextLen = (prefixMarkdown + suffixMarkdown).length
+  const ocrContextLen = requestPrefix.length - prefixMarkdown.length
+  const totalWithOcr = totalTextLen + ocrContextLen
+  
+  const overLimit = totalWithOcr > SIZE_LIMIT
 
-  if (DEBUG) {
-    console.log('[Copilot] ===== LLM Request =====')
-    console.log('[Copilot] PREFIX:', requestPrefix)
-    console.log('[Copilot] SUFFIX:', suffixMarkdown)
-    console.log('[Copilot] ======================')
+  if (overLimit) {
+    setCopilotEnabled(view, false)
+    return
   }
 
   if (runtime.debounceTimer) {