[contexto]: BufferStore abstraction for context engine message buffer

[DaevMithran]

Problem

The context engine holds two pieces of in-memory state in CompactionState that are tightly coupled to raw data structures:

1. bufferedMessages: WebhookPayload[] - episode payloads accumulated after each turn, flushed to backend on compact/dispose. Lost on process crash.
2. injectedItemIds: Set<string> - tracks which search result IDs have been injected into context for deduplication. Lost on crash, causing duplicate context injection after restart.

Both live as plain arrays/sets with no abstraction boundary, making it impossible to swap in a persistent backend without rewriting both engine implementations.

Proposal

Introduce a BufferStore interface that unifies both concerns behind a single abstraction, and an InMemoryBufferStore that preserves current behavior. Both DefaultContextEngine and SlidingWindowEngine migrate from direct array/set manipulation to the BufferStore API.

interface BufferStore {
  // Episode buffer
  push(payload: WebhookPayload, sessionId: string, sessionKey: string): void;
  getAll(): BufferedEntry[];
  count(): number;
  clear(): void;
  deleteByRowids(rowids: number[]): void;

  // Dedup tracking
  hasInjectedId(id: string): boolean;
  addInjectedId(id: string): void;
  clearInjectedIds(): void;

  // Lifecycle
  close(): void;
}

Scope

• New buffer-store.ts with BufferStore interface, BufferedEntry type, and InMemoryBufferStore
• Replace bufferedMessages + injectedItemIds in CompactionState with single buffer: BufferStore
• Migrate both engines and base class to use the buffer API

Non-goals

• SQLite persistence (future follow-up: SqliteBufferStore implementing the same interface, following lossless-claw's node:sqlite DatabaseSync patterns)
• Config changes or new dependencies