feat(#41): Open Cowriting Review Panel from markdown file/tab right-click

The review preview (`cowriting.showTrackChangesPreview`) was only reachable from
the command palette and `ctrl+alt+r` — neither where a writer's hand naturally
goes. Add the obvious right-click entry points to the plugin's central surface:

- `explorer/context` + `editor/title/context` menu items gated on
  `resourceLangId == markdown`, both invoking `showTrackChangesPreview`.
- The command now resolves the *clicked* document: it opens the passed Uri if
  it isn't already an open document (the Explorer case), instead of falling back
  to the active editor. No-arg invocation (palette / keybinding) is unchanged.
- Retitle the command to "Open Cowriting Review Panel" so the menus read the
  operator's wording (palette shows "Cowriting: Open Cowriting Review Panel").

E2E: clicked-doc resolution (open + not-yet-open + no-arg fallback), both menu
contributions present + markdown-gated, title, and keybinding unchanged.

Closes #41.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/trackChangesPreview.ts

@@ -71,9 +71,16 @@ export class TrackChangesPreviewController implements vscode.Disposable {
     this.disposables.push(
       // F11 (SLICE-5): the editor/title gateway passes the tab's resource Uri;
       // the palette / keybinding pass nothing → fall back to the active editor.
-      vscode.commands.registerCommand("cowriting.showTrackChangesPreview", (uri?: vscode.Uri) => {
-        const byUri = uri && vscode.workspace.textDocuments.find((d) => d.uri.toString() === uri.toString());
-        this.show(byUri || vscode.window.activeTextEditor?.document);
+      // #41: the explorer/tab right-click also passes the clicked Uri, which may
+      // not be an open document yet — open it so we preview the clicked file, not
+      // whatever happens to be the active editor.
+      vscode.commands.registerCommand("cowriting.showTrackChangesPreview", async (uri?: vscode.Uri) => {
+        if (uri) {
+          const open = vscode.workspace.textDocuments.find((d) => d.uri.toString() === uri.toString());
+          this.show(open ?? (await vscode.workspace.openTextDocument(uri)));
+          return;
+        }
+        this.show(vscode.window.activeTextEditor?.document);
       }),
       // F11: document-scoped Ask-Claude (also reused by #42's gateway). Edits the
       // active markdown doc; the rewrite is diffed into per-hunk F4 proposals.