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>

package.json

@@ -85,8 +85,8 @@
         "category": "Cowriting"
       },
       {
-        "command": "cowriting.pinDiffBaseline",
-        "title": "Cowriting: Pin Review Baseline to Now",
+        "command": "cowriting.markReviewed",
+        "title": "Mark Changes as Reviewed",
         "category": "Cowriting"
       },
       {
@@ -149,7 +149,7 @@
           "when": "false"
         },
         {
-          "command": "cowriting.pinDiffBaseline",
+          "command": "cowriting.markReviewed",
           "when": "editorLangId == markdown"
         },
         {