feat: Enable file editing and saving (#252)

## Summary
- Adds file writing capability to Monaco editor in the file viewer
- Implements writeFile API on the server for workspace files
- Integrates save functionality into the file viewer UI with proper
state management

## Bug Fixes (Review Feedback)
- Fixed failed save discarding edits when switching files - now checks
save result and only proceeds if successful
- Fixed refresh overwriting dirty editor state - now prompts for
confirmation before discarding edits
- Fixed save button unable to save empty files - changed check from `if
(content)` to `if (content !== undefined && content !== null)`
- Added agent edit conflict detection - when agent edits file while user
has unsaved changes, shows conflict dialog with Overwrite/Cancel options
- Fixed dialog appearing behind unpinned sidebar - increased alert
dialog z-index to z-100

## Related Issues
- Closes #251

---------

Co-authored-by: Jess Chadwick <jchadwick@gmail.com>

packages/ui/src/components/file-viewer/monaco-file-viewer.tsx

@@ -9,6 +9,8 @@ interface MonacoFileViewerProps {
   scopeKey: string
   path: string
   content: string
+  onSave?: (content: string) => void
+  onContentChange?: (content: string) => void
 }
 
 export function MonacoFileViewer(props: MonacoFileViewerProps) {
@@ -33,6 +35,11 @@ export function MonacoFileViewer(props: MonacoFileViewerProps) {
     editor = null
   }
 
+  const saveContent = () => {
+    if (!editor || !props.onSave) return
+    props.onSave(editor.getValue())
+  }
+
   onMount(() => {
     let cancelled = false
     void (async () => {
@@ -44,7 +51,7 @@ export function MonacoFileViewer(props: MonacoFileViewerProps) {
       editor = monaco.editor.create(host, {
         value: "",
         language: "plaintext",
-        readOnly: true,
+        readOnly: false,
         automaticLayout: true,
         lineNumbers: "on",
         minimap: { enabled: false },
@@ -54,6 +61,14 @@ export function MonacoFileViewer(props: MonacoFileViewerProps) {
         fontSize: 13,
       })
 
+      editor.addCommand(monaco.KeyMod.CtrlCmd | monaco.KeyCode.KeyS, saveContent)
+
+      editor.onDidChangeModelContent(() => {
+        if (props.onContentChange) {
+          props.onContentChange(editor.getValue())
+        }
+      })
+
       setReady(true)
     })()