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/web/src/api/files.ts

@@ -110,7 +110,7 @@ export const createWebFilesAPI = (): 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 response = await fetch(`/api/fs/stat?path=${encodeURIComponent(target)}`);
 
@@ -124,6 +124,7 @@ export const createWebFilesAPI = (): FilesAPI => ({
       path: typeof (result as { path?: string }).path === 'string' ? normalizePath((result as { path: string }).path) : target,
       isFile: Boolean((result as { isFile?: boolean }).isFile),
       size: typeof (result as { size?: number }).size === 'number' ? (result as { size: number }).size : 0,
+      mtimeMs: typeof (result as { mtimeMs?: number }).mtimeMs === 'number' ? (result as { mtimeMs: number }).mtimeMs : undefined,
     };
   },