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>

README.md

@@ -245,7 +245,10 @@ A clean, **zero-annotation editor** on the left; the rendered preview on the
 right as the **single interactive review surface**. The editor carries no
 attribution tint, no in-editor proposal threads, and no diff — all review lives
 in the preview, toggled by the **Annotations** switch in its header (on by
-default). `Ctrl+Alt+R` opens **"Open Review Preview"**.
+default). Open it via `Ctrl+Alt+R`, the editor title-bar button, or
+**right-click a markdown file in the Explorer / its editor tab →
+"Open Cowriting Review Panel"** (#41) — the right-click acts on the clicked
+document, opening it first if needed.
 
 In the on-state the preview shows **green = human / blue = Claude /
 strikethrough = deleted**, and surfaces each of Claude's pending F4 proposals as