feat!: sunset the review-panel webview — built-in preview + native diff are the review surfaces (D3/D17, spec §6.10); Keep/Reject lens copy (PUC-2)

Deletes the last bespoke UI surface (TrackChangesPreviewController + its
sealed webview client assets); every entry point re-points at native VS
Code chrome per spec §5: the #41 right-click entries become
cowriting.openReviewPreview (enter coediting if needed -> "Open Preview to
the Side"), Ctrl+Alt+R/Cmd+Alt+R moves to cowriting.reviewChanges (native
diff), and the F12 CodeLens per-proposal titles read "Keep"/"Reject" with
a top-of-file "Keep all (N)"/"Reject all" pair once >=2 proposals are
pending. EditFlow drops its own askClaude/askEditInstruction (the
webview's only caller) and its now-unused constructor params.

Coverage that lived only in the webview's test seams (renderHtmlFor,
receiveMessage, isOpen, ...) moves to direct calls against the surviving
controllers/pure renderReview (test/e2e/suite/helpers.ts gains a shared
renderHtmlFor probe); a genuine gap (pending-proposal + unchanged-block
rendering) is backfilled in test/previewAnnotations.test.ts. README's "how
it works" is rewritten as the native-surface map; the superseded F6/F7/F9/
F10/F11 sections are kept as a marked historical record rather than
deleted outright.

292 unit + 91 E2E green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -109,8 +109,8 @@
         "icon": "$(git-compare)"
       },
       {
-        "command": "cowriting.showTrackChangesPreview",
-        "title": "Open Cowriting Review Panel",
+        "command": "cowriting.openReviewPreview",
+        "title": "Open Cowriting Review Preview",
         "category": "Cowriting"
       },
       {
@@ -252,7 +252,7 @@
           "group": "navigation@4"
         },
         {
-          "command": "cowriting.showTrackChangesPreview",
+          "command": "cowriting.openReviewPreview",
           "when": "editorLangId == markdown",
           "group": "navigation@9"
         }
@@ -264,14 +264,14 @@
           "group": "1_cowriting@1"
         },
         {
-          "command": "cowriting.showTrackChangesPreview",
+          "command": "cowriting.openReviewPreview",
           "when": "resourceLangId == markdown",
           "group": "1_cowriting@3"
         }
       ],
       "explorer/context": [
         {
-          "command": "cowriting.showTrackChangesPreview",
+          "command": "cowriting.openReviewPreview",
           "when": "resourceLangId == markdown",
           "group": "navigation@9"
         }
@@ -330,10 +330,10 @@
     },
     "keybindings": [
       {
-        "command": "cowriting.showTrackChangesPreview",
+        "command": "cowriting.reviewChanges",
         "key": "ctrl+alt+r",
         "mac": "cmd+alt+r",
-        "when": "editorLangId == markdown"
+        "when": "editorLangId == markdown && cowriting.isCoediting"
       },
       {
         "command": "cowriting.edit",