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/server/src/filesystem/browser.ts

@@ -81,6 +81,14 @@ export class FileSystemBrowser {
     return { path: relativePath, absolutePath }
   }
 
+  writeFile(relativePath: string, contents: string): void {
+    if (this.unrestricted) {
+      throw new Error("writeFile is not available in unrestricted mode")
+    }
+    const resolved = this.toRestrictedAbsolute(relativePath)
+    fs.writeFileSync(resolved, contents, "utf-8")
+  }
+
   readFile(relativePath: string): string {
     if (this.unrestricted) {
       throw new Error("readFile is not available in unrestricted mode")