feat(ux): unify Ask-Claude-to-Edit on a split-below multi-line webview

Operator feedback iteration on the Ask-Claude input. Replaces the inline
comment-thread box (and the top-center QuickInput before it) with ONE input for
both scopes: a multi-line webview in a split pane below the document.

- New `editInstructionInput.ts` (`promptEditInstruction`): a tall, resizable
  textarea + Send, opened via `newGroupBelow` so it sits under the document, not
  over it. Auto-focused; ⌘↵/Ctrl↵ sends; Esc cancels. Cancel/Esc confirms ONLY
  when there's text to lose (a webview can read its own textarea); closing the tab
  is an explicit dismiss. On submit/cancel the split collapses AND focus is handed
  back to the document, so the bottom panel (Output/Debug Console) no longer pops.
- Selection edits use the same box; the document above keeps the selection
  highlighted (inactive-selection style) so the user sees what Claude will edit —
  the selection is captured before the prompt, never touched.
- Removes the whole inline-comment mechanism: deletes `inlineAsk.ts`
  (InlineAskController), the `cowriting.askClaude.submit`/`.cancel` commands, their
  comment-thread menus, and the Escape keybinding — net deletion. One overridable
  seam `TrackChangesPreviewController.askEditInstruction` (used by editSelection
  and the preview), stubbed by the host E2E.

Sealed webview (INV-8/35): collects text only, no SDK/secret surface, no network.
242 unit + typecheck (src + e2e) + build green.

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

package.json

@@ -64,16 +64,6 @@
         "title": "Ask Claude to Edit",
         "category": "Cowriting"
       },
-      {
-        "command": "cowriting.askClaude.submit",
-        "title": "Ask Claude to Edit",
-        "category": "Cowriting"
-      },
-      {
-        "command": "cowriting.askClaude.cancel",
-        "title": "Cancel",
-        "category": "Cowriting"
-      },
       {
         "command": "cowriting.editSelection",
         "title": "Ask Claude to Edit Selection",
@@ -133,14 +123,6 @@
           "command": "cowriting.rejectProposal",
           "when": "false"
         },
-        {
-          "command": "cowriting.askClaude.submit",
-          "when": "false"
-        },
-        {
-          "command": "cowriting.askClaude.cancel",
-          "when": "false"
-        },
         {
           "command": "cowriting.pinDiffBaseline",
           "when": "editorLangId == markdown"
@@ -205,11 +187,6 @@
           "command": "cowriting.reply",
           "group": "inline",
           "when": "commentController == cowriting.threads"
-        },
-        {
-          "command": "cowriting.askClaude.submit",
-          "group": "inline",
-          "when": "commentController == cowriting.askClaude"
         }
       ],
       "comments/commentThread/title": [
@@ -222,11 +199,6 @@
           "command": "cowriting.reopenThread",
           "group": "inline",
           "when": "commentController == cowriting.threads && commentThread =~ /^resolved$/"
-        },
-        {
-          "command": "cowriting.askClaude.cancel",
-          "group": "inline",
-          "when": "commentController == cowriting.askClaude"
         }
       ]
     },