feat: baseline router — git HEAD + snapshot per INV-7/D13/D14; retire machine-landing advance (spec §6.4)

Task 2 of the native-surfaces migration plan. GitBaselineAdapter resolves a
document's HEAD blob via the built-in vscode.git extension (hardened with a
.git/logs/HEAD watch that nudges repo.status(), since the git extension's own
watcher doesn't reliably notice out-of-band commits on non-workspace-folder
repos in the E2E host). DiffViewController is reworked into a baseline router:
head mode (git-tracked, never persisted, re-read on commit) vs snapshot mode
(captured on CoeditingRegistry entry, re-pinned by "Mark Changes as Reviewed"
— renamed from cowriting.pinDiffBaseline, D14). The shipped machine-landing
baseline advance (#48/INV-18) is retired: a landed Claude edit now stays a
visible change-since-baseline until commit or review (INV-7/D21). Legacy
on-disk reasons ("opened"/"machine-landing") migrate to "entered" via
BaselineStore.normalizeReason on load.

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

src/trackChangesPreview.ts

@@ -418,15 +418,15 @@ export class TrackChangesPreviewController implements vscode.Disposable {
   }
 
   private epochLabel(baseline: { reason: string; capturedAt: string } | undefined): string {
-    if (!baseline) return "opened (no baseline yet)";
+    if (!baseline) return "not coediting (no baseline yet)";
     const time = new Date(baseline.capturedAt).toLocaleTimeString();
     switch (baseline.reason) {
-      case "machine-landing":
-        return `Claude landed ${time}`;
+      case "head":
+        return `HEAD ${time}`;
       case "pinned":
-        return `pinned ${time}`;
+        return `reviewed ${time}`;
       default:
-        return `opened ${time}`;
+        return `entered ${time}`;
     }
   }