F6 #34: delete the dead two-pane diff-view UI, keep the baseline data layer

F10 (#29) made the rendered preview the single review surface and hid the F6
two-pane vscode.diff view (command + ctrl+alt+d set when:false). This removes
that now-unreachable view code:

- DiffViewController: drop toggle/findDiffTab/epochLabel/isDiffOpen, the
  `cowriting-baseline:` TextDocumentContentProvider + BASELINE_SCHEME + baselineUri
  + the content-provider change emitter, and the toggleDiffView command. The
  baseline DATA layer is fully intact — ensureBaseline/advance/pin/capture,
  getBaseline, baselineFilePath, onDidChangeBaseline, persistence (INV-19), and
  the machine-landing auto-advance (INV-18) that F7/F10 consume.
- package.json: remove the toggleDiffView command, its commandPalette entry, and
  the ctrl+alt+d keybinding.
- E2E: diffView suite keeps the baseline-data-layer tests, drops the two-pane
  view tests; the F10 + no-workspace suites assert toggleDiffView is now absent
  (was: declared-but-hidden).

Deliberate deviation from the issue's literal acceptance: pinDiffBaseline is
KEPT. The canonical Solution Design (coauthoring-interactive-review.md §6.7)
scopes the removal to the two-pane VIEW only ("keep the controller + baseline
store"); pin() lives in the baseline lifecycle (§6.4), never touches vscode.diff,
and is exercised by live F7 baseline-reset tests. Where the P3 capture draft and
the approved spec conflict, the spec wins (documentation-leads-automation).

194 unit + 49 E2E green; typecheck + build clean. No F7/F10 behavior change.

Closes #34

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

package.json

@@ -69,14 +69,9 @@
         "title": "Propose Agent Edit (internal seam)",
         "category": "Cowriting"
       },
-      {
-        "command": "cowriting.toggleDiffView",
-        "title": "Cowriting: Toggle Diff View",
-        "category": "Cowriting"
-      },
       {
         "command": "cowriting.pinDiffBaseline",
-        "title": "Cowriting: Pin Diff Baseline to Now",
+        "title": "Cowriting: Pin Review Baseline to Now",
         "category": "Cowriting"
       },
       {
@@ -103,10 +98,6 @@
           "command": "cowriting.rejectProposal",
           "when": "false"
         },
-        {
-          "command": "cowriting.toggleDiffView",
-          "when": "false"
-        },
         {
           "command": "cowriting.pinDiffBaseline",
           "when": "false"
@@ -145,11 +136,6 @@
       ]
     },
     "keybindings": [
-      {
-        "command": "cowriting.toggleDiffView",
-        "key": "ctrl+alt+d",
-        "when": "false"
-      },
       {
         "command": "cowriting.showTrackChangesPreview",
         "key": "ctrl+alt+r",