Add MIGRATION.md documenting the sqlite_crdt -> crdt_sync move

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019KwUCPESvE5CjCtmUFz8xQ

MIGRATION.md

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+# Migration: `sqlite_crdt` → shared `crdt_sync`
+
+todo's note storage and sync were moved off `sqlite_crdt` onto the shared
+[`crdt_sync`](https://github.com/kuhyx/utils/tree/main/crdt_sync_dart) library
+(pinned tag `crdt_sync_dart-v0.3.0`), so every companion app now syncs through
+one library.
+
+## What changed
+
+- **Storage:** `NoteRepository` is backed by `crdt_sync`'s `LogStore` (a
+  `Log = Map<String, Record>` persisted as JSON). Each `Note` is stored *as* a
+  `Record` (per-field last-writer-wins + a sticky tombstone), so the on-disk
+  log **is** the sync wire format — no separate adapter. Filtering/sorting run
+  in Dart over the in-memory log.
+- **Sync:** `SyncService` pushes each device's `Log` to
+  `todo-sync/notes/<nodeId>.json` in `kuhyx/syncs` (new dir; the old
+  `changesets/` format is incompatible, so it's a clean cutover). It uses
+  `crdt_sync`'s `GitHubClient` + `logToJson`/`logFromJson`.
+- **Client:** todo's local `lib/sync/github_client.dart` was deleted; it now
+  uses `crdt_sync`'s `GitHubClient` (which gained `canAccessRepo()` +
+  `deleteFile()` in v0.3.0). `putFileText` self-resolves the file sha, so the
+  old SHA-tracking is gone.
+
+## The one-time on-device migration
+
+`NoteRepository.open()` migrates a pre-existing `sqlite_crdt` DB into the log on
+first launch, guarded by a persisted `crdt.migratedFromSqlite` flag. Verified
+on-device: a real 27-note backlog (+5 tombstones) migrated intact.
+
+`sqlite_crdt` is **retained as a dependency** only to read that legacy DB during
+migration — the old `todo.db` is left untouched as a safety net. Remove the dep
+in a later cleanup once every device is known-migrated.
+
+## Surprises / non-obvious decisions
+
+- **Seed HLCs from each note's `updatedAt`, not "now".** Two devices migrate
+  independently; stamping migration-time "now" would let per-field LWW resolve
+  a pre-cutover edit arbitrarily. Seeding from `updatedAt` keeps
+  last-real-edit-wins across the cutover.
+- **Reuse the legacy `sqlite_crdt` node id** (persisted in prefs) so a device
+  keeps its `devices/<id>` identity and devices still converge.
+- **`watchNotes`/`watchCount` seed in `onListen`**, not lazily in an `async*`
+  body — otherwise a write between `listen()` and the first generator tick
+  swallows the initial snapshot.