storage: introduce StorageAdapter seam + FileAdapter refactor (Phase 1 of #139)

[willwashburn]

Phase 1 of #139.

Goal

Introduce a StorageAdapter interface inside @relayburn/ledger and refactor the existing filesystem code to live behind a FileAdapter implementation. Zero behavior change — same JSONL output, same sidecars, same locks, byte-identical artifacts. This phase is purely the seam; no new adapter ships.

This is the foundation every other phase builds on, and the phase with the largest regression surface (it touches writer.ts, reader.ts, content.ts, lock.ts, archive.ts).

Interface

packages/ledger/src/adapters/adapter.ts:

export interface StorageAdapter {
  readonly kind: 'file' | 'sqlite' | 'postgres' | 'http';

  appendTurns(turns: TurnRecord[]): Promise<void>;
  appendCompactions(events: CompactionEvent[]): Promise<void>;
  appendRelationships(records: SessionRelationshipRecord[]): Promise<void>;
  appendToolResultEvents(events: ToolResultEventRecord[]): Promise<void>;
  appendUserTurns(records: UserTurnRecord[]): Promise<void>;
  appendStamp(stamp: StampLine): Promise<void>;
  appendContent(records: ContentRecord[]): Promise<void>;

  queryTurns(q: Query): AsyncIterable<EnrichedTurn>;
  queryCompactions(q: Query): AsyncIterable<CompactionEvent>;
  queryRelationships(q: Query): AsyncIterable<SessionRelationshipRecord>;
  queryToolResultEvents(q: Query): AsyncIterable<ToolResultEventRecord>;
  queryUserTurns(q: Query): AsyncIterable<UserTurnRecord>;
  readContent(selector: ReadContentSelector): AsyncIterable<ContentLine>;
  listContentSessionIds(): Promise<string[]>;
  pruneContent(opts: PruneOptions): Promise<PruneResult>;

  withLock<T>(name: string, fn: () => Promise<T>): Promise<T>;

  init(): Promise<void>;
  close(): Promise<void>;
}

packages/ledger/src/adapters/factory.ts exposes a process-singleton getAdapter() that resolves from RELAYBURN_STORAGE (defaults to file).

Refactor

• New: packages/ledger/src/adapters/{adapter,factory,file-adapter}.ts.
• Modified: the public functions in writer.ts, reader.ts, content.ts, lock.ts become 1-line wrappers delegating to getAdapter(). Their internal helpers (appendLines, streamLines, loadIndex, FS lock impl) move into FileAdapter.
• Modified: archive.ts becomes FileAdapter-internal — it's a JSONL-specific read cache and isn't part of the cross-adapter contract. Its public functions (buildArchive, openArchive, getArchiveStatus, rebuildArchive) stay re-exported for backwards compat but become no-ops on non-file adapters (relevant only in later phases).
• Reused unchanged: index-sidecar.ts (pure dedup-hash functions used by every adapter), schema.ts (record types), cursors.ts/hwm.ts/plans.ts/config.ts (host-local state, stays on the filesystem).
• Modified: index.ts re-exports StorageAdapter, getAdapter, the adapter-kind type.

Verification

• All existing pnpm test suites (ledger.test.ts, content.test.ts, lock.test.ts, archive.test.ts, cursors.test.ts, plans.test.ts) pass unchanged — they exercise the FileAdapter through the unchanged public API.
• relayburn ingest against a real ~/.claude directory produces a byte-identical ledger.jsonl and content/ tree before vs. after the refactor.
• relayburn analyze --json returns identical output.