fix(files): refresh open file content after external changes (#967)

* fix(files): refresh open file content after external edits

Previously, opening a file in the Files view and then editing it externally
(e.g. via CLI or another editor) would show stale content. Even closing and
reopening the file returned cached content — a full page reload was required.

Root causes:
1. The in-memory readFile cache used path-only hits, with no metadata
   validation. External edits were invisible until the cache was evicted.
2. No polling mechanism existed to detect external changes to the open file.

Fix:
- Add mtimeMs to statFile across all runtimes (web, VS Code, desktop).
- Cache layer (RuntimeAPIProvider): validate cache hits against current stat
  metadata (mtimeMs + size). On miss, use stat→read→stat to avoid TOCTOU.
- UI layer (FilesView): poll the open file every 2s; on detected change, set
  loadedFilePath=null to trigger the existing load effect once (no double
  reload). Skip polling when tab is hidden or editor has unsaved changes.
- After save, refresh the stat ref so the next poll doesn't see a spurious
  change from the save itself.

Addresses review feedback from PR #827 (double reload + TOCTOU).

* fix(files): address P2 review findings

- readFreshFile retry now uses stat→read→stat to maintain TOCTOU
  protection during the retry path (not just the initial read).
- Replace isDirty in polling effect deps with isDirtyRef to avoid
  unnecessary interval teardown/restart on every edit/save cycle.

packages/vscode/webview/api/files.ts

@@ -71,13 +71,14 @@ export const createVSCodeFilesAPI = (): FilesAPI => ({
     };
   },
 
-  async statFile(path: string): Promise<{ path: string; isFile: boolean; size: number }> {
+  async statFile(path: string): Promise<{ path: string; isFile: boolean; size: number; mtimeMs?: number }> {
     const target = normalizePath(path);
-    const data = await sendBridgeMessage<{ path?: string; isFile?: boolean; size?: number }>('api:fs:stat', { path: target });
+    const data = await sendBridgeMessage<{ path?: string; isFile?: boolean; size?: number; mtimeMs?: number }>('api:fs:stat', { path: target });
     return {
       path: typeof data?.path === 'string' ? normalizePath(data.path) : target,
       isFile: Boolean(data?.isFile),
       size: typeof data?.size === 'number' ? data.size : 0,
+      mtimeMs: typeof data?.mtimeMs === 'number' ? data.mtimeMs : undefined,
     };
   },