feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API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Tests: superseded suites deleted; fork/commit blocks excised from shared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/migration/MigrationRunner.ts

@@ -192,7 +192,7 @@ export class MigrationRunner {
    * Entity-level errors are tracked (not thrown). If maxErrors is exceeded, migration
    * stops early and returns partial results with errors.
    */
-  async run(options?: Pick<MigrateOptions, 'onProgress' | 'maxErrors'>): Promise<Omit<MigrationResult, 'backupBranch'>> {
+  async run(options?: Pick<MigrateOptions, 'onProgress' | 'maxErrors'>): Promise<Omit<MigrationResult, 'backupPath'>> {
     const state = await this.getState()
     const pending = this.getPendingMigrations(state)
 
@@ -259,93 +259,6 @@ export class MigrationRunner {
     }
   }
 
-  /**
-   * Run specific migrations without checking completion state.
-   * Used for branch iterations where the state on main says "completed"
-   * but branch-local entities still need transforming.
-   *
-   * Safe because transforms are idempotent (return null when already applied).
-   * Does NOT save migration state — the authoritative state lives on main.
-   */
-  async runMigrations(
-    migrations: Migration[],
-    options?: Pick<MigrateOptions, 'onProgress' | 'maxErrors'>
-  ): Promise<Omit<MigrationResult, 'backupBranch'>> {
-    if (migrations.length === 0) {
-      return { migrationsApplied: [], entitiesProcessed: 0, entitiesModified: 0, errors: [] }
-    }
-
-    let totalProcessed = 0
-    let totalModified = 0
-    const appliedMigrations: string[] = []
-    const errors: MigrationError[] = []
-    const maxErrors = options?.maxErrors ?? DEFAULT_MAX_ERRORS
-    const batchConfig = this.storage.getBatchConfig()
-    const batchSize = batchConfig.maxBatchSize
-    const batchDelay = batchConfig.batchDelayMs
-
-    for (const migration of migrations) {
-      if (errors.length >= maxErrors) break
-
-      let processed = 0
-      let modified = 0
-
-      if (migration.applies === 'nouns' || migration.applies === 'both') {
-        const result = await this.migrateNouns(migration, 0, batchSize, batchDelay, errors, maxErrors, options?.onProgress)
-        processed += result.processed
-        modified += result.modified
-      }
-
-      if (migration.applies === 'verbs' || migration.applies === 'both') {
-        if (errors.length < maxErrors) {
-          const result = await this.migrateVerbs(migration, 0, batchSize, batchDelay, errors, maxErrors, options?.onProgress)
-          processed += result.processed
-          modified += result.modified
-        }
-      }
-
-      totalProcessed += processed
-      totalModified += modified
-      if (modified > 0) {
-        appliedMigrations.push(migration.id)
-      }
-    }
-
-    return {
-      migrationsApplied: appliedMigrations,
-      entitiesProcessed: totalProcessed,
-      entitiesModified: totalModified,
-      errors
-    }
-  }
-
-  /**
-   * Clean up old system:backup branches created by previous migrations.
-   * Identifies backups via ref metadata (not by name prefix).
-   */
-  async cleanupOldBackups(): Promise<void> {
-    const refManager = this.storage.refManager
-    if (!refManager) return
-
-    const refs = await refManager.listRefs()
-    const currentBranch = this.storage.currentBranch || 'main'
-
-    for (const ref of refs) {
-      if (
-        ref.metadata?.type === 'system:backup' &&
-        ref.name.startsWith('refs/heads/') &&
-        ref.name !== `refs/heads/${currentBranch}`
-      ) {
-        const branchName = ref.name.replace('refs/heads/', '')
-        try {
-          await refManager.deleteRef(branchName)
-        } catch {
-          // Ignore — branch may be current or protected
-        }
-      }
-    }
-  }
-
   // ─── Private helpers ───────────────────────────────────────────────
 
   private async migrateNouns(
@@ -524,12 +437,11 @@ export class MigrationRunner {
 
   private async saveResumeState(migrationId: string, offset: number): Promise<void> {
     const state = await this.getState()
-    const branch = this.storage.currentBranch || 'main'
     await this.saveState({
       completedVersion: state?.completedVersion || '',
       completedAt: state?.completedAt || 0,
       completedMigrations: state?.completedMigrations || [],
-      resumeState: { migrationId, lastProcessedOffset: offset, branch }
+      resumeState: { migrationId, lastProcessedOffset: offset }
     })
   }