feat(f11): SLICE-1 — Pin baseline toolbar button + reachability (#43)

Homes the orphaned cowriting.pinDiffBaseline command and gives the writer a
reachable Pin control in the preview toolbar. Per spec §7.2 SLICE-1
(docs/superpowers/specs/2026-06-12-f11-preview-toolbar-interaction-surface.md).

- trackChangesPreview: extract onDidReceiveMessage into handleWebviewMessage;
  add the F11 `pinBaseline` intent → DiffViewController.pin(previewedDoc) (the
  bound doc, not activeTextEditor — §6.7); ToolbarMsg union; receiveMessage test
  seam exercising the real message→seam wiring (INV-35).
- webview: ⌖ Pin baseline button in #cw-header posting { type: "pinBaseline" };
  theme-aware toolbar-button CSS (light/dark/high-contrast, disabled state).
- package.json: unhide pinDiffBaseline — commandPalette `when` false →
  editorLangId == markdown (resolves the #34 orphan from the command side).
- host E2E (test/e2e/suite/f11Toolbar.test.ts): pinBaseline message clears the
  change-marks + advances the baseline to `pinned`; palette `when` is reachable.

Also archives the F11 implementation plan to docs/superpowers/plans/.

197 unit + 47 host E2E green.

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

package.json

@@ -100,7 +100,7 @@
         },
         {
           "command": "cowriting.pinDiffBaseline",
-          "when": "false"
+          "when": "editorLangId == markdown"
         }
       ],
       "editor/context": [