feat(git-sync): thin-meta phase 1 — the .gitmost/index.json sidecar module

Pure read/write/lookup for the vault sidecar index that will hold page identity
(pageId) + collision token (slugId) keyed by file path, so the .md files can be
clean markdown. parseVaultIndex is tolerant (missing/garbage/bad entries degrade
to empty/skipped — never crashes a cycle); serializeVaultIndex is deterministic
(sorted keys -> stable diffs, no churn). Lookups (pageIdAt, pathForPageId reverse,
trackedPageIds) + mutations (set/remove/move). NOT wired into pull/push yet — no
behavior change. 5 unit tests; engine suite green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/git-sync/src/index.ts

@@ -119,3 +119,18 @@ export type {
   RunCycleResult,
   CycleFs,
 } from "./engine/cycle";
+
+export {
+  VAULT_INDEX_PATH,
+  emptyVaultIndex,
+  parseVaultIndex,
+  serializeVaultIndex,
+  pageIdAt,
+  slugIdAt,
+  pathForPageId,
+  trackedPageIds,
+  setEntry,
+  removeAt,
+  moveEntry,
+} from "./engine/vault-index";
+export type { VaultIndex, VaultIndexEntry } from "./engine/vault-index";