feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API

The COW version-control surface (fork, branches, checkout, commit,
getHistory/streamHistory, asOfCommit, brain.versions) is gone, along with
its subsystems: src/versioning/, the COW object store (CommitLog,
CommitObject, RefManager, TreeObject), HistoricalStorageAdapter, and the
TypeAwareHNSWIndex path it kept alive. The Db API (now/transact/asOf/
with/persist/restore) is the one versioning model in 8.0.

Survivors and replacements:
- BlobStorage survives (the VFS stores file content through it), relocated
  to src/storage/blobStorage.ts with binaryDataCodec.ts; its adapter
  interface is now BlobStoreAdapter, slimmed to the consumed surface
  (write/read/has/delete/getMetadata + MIME-aware compression policy).
- brain.migrate() backup branches are replaced by persist-before-migrate:
  MigrateOptions.backupTo persists a hard-link snapshot of the current
  generation before any transform runs; MigrationResult.backupPath reports
  it, and brain.restore(path) brings it back wholesale.
- CLI: cow.ts (fork/branch/checkout/history/migrate) is replaced by
  snapshot.ts — snapshot <path>, restore <path>, history (tx-log),
  generation.
- New public read API: brain.transactionLog({limit}) exposes the reified
  tx-log (generation/timestamp/meta, newest first) that backs the CLI
  history command; TxLogEntry is exported.

Tests: superseded suites deleted; fork/commit blocks excised from shared
suites; BlobStorage tests relocated + reworked against the slimmed store;
migration tests now prove the backupTo snapshot/restore round trip; new
transactionLog coverage in db-mvcc.

src/types/brainy.types.ts

@@ -1099,7 +1099,6 @@ export interface BrainyConfig {
      *  including plugin-provided factories (e.g. Cortex mmap). */
     rootDirectory?: string
     options?: any
-    branch?: string              // COW branch name (default: 'main')
   }
 
   // Index configuration

src/types/brainyInterface.ts

@@ -164,7 +164,8 @@ export interface BrainyInterface<T = unknown> {
   /**
    * Run pending data migrations, or preview what would change.
    *
-   * @param options - Pass { dryRun: true } to preview without writing
+   * @param options - Pass `{ dryRun: true }` to preview without writing;
+   *   pass `{ backupTo: path }` to persist a pre-migration snapshot.
    * @returns Migration result or preview depending on options
    *
    * @example
@@ -173,9 +174,9 @@ export interface BrainyInterface<T = unknown> {
    * const preview = await brain.migrate({ dryRun: true })
    * console.log(preview.affectedEntities)
    *
-   * // Apply
-   * const result = await brain.migrate()
-   * console.log(result.backupBranch) // 'pre-migration-7.17.0'
+   * // Apply with a restore point
+   * const result = await brain.migrate({ backupTo: '/backups/pre-migration' })
+   * console.log(result.backupPath) // '/backups/pre-migration'
    * ```
    */
   migrate(options?: MigrateOptions): Promise<MigrationResult | MigrationPreview>