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/workspaces/manager.ts

@@ -83,6 +83,12 @@ export class WorkspaceManager {
     }
   }
 
+  writeFile(workspaceId: string, relativePath: string, contents: string): void {
+    const workspace = this.requireWorkspace(workspaceId)
+    const browser = new FileSystemBrowser({ rootDir: workspace.path })
+    browser.writeFile(relativePath, contents)
+  }
+
   async create(folder: string, name?: string): Promise<WorkspaceDescriptor> {
  
     const id = `${Date.now().toString(36)}`