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>

package.json

@@ -76,7 +76,7 @@
       },
       {
         "command": "cowriting.showTrackChangesPreview",
-        "title": "Cowriting: Open Review Preview",
+        "title": "Open Cowriting Review Panel",
         "category": "Cowriting"
       },
       {
@@ -119,6 +119,20 @@
           "group": "navigation@9"
         }
       ],
+      "editor/title/context": [
+        {
+          "command": "cowriting.showTrackChangesPreview",
+          "when": "resourceLangId == markdown",
+          "group": "1_cowriting"
+        }
+      ],
+      "explorer/context": [
+        {
+          "command": "cowriting.showTrackChangesPreview",
+          "when": "resourceLangId == markdown",
+          "group": "navigation@9"
+        }
+      ],
       "editor/context": [
         {
           "command": "cowriting.editSelection",