Refactor: split agent-storage.ts (1262 LOC) per storage domain

[serge-ivo]

Follow-up to the largest-files audit (#125). workers/api/src/agent-storage.ts (1262 LOC) spans collections, files, vectors, activity, summaries, and memory in one class.

Task

Split per domain (collections/files/vectors/activity/summaries/memory), each a focused module the engine composes.

Constraints

• Behavior-preserving; keep storage + storage-tools tests green; same public method surface.

[serge-ivo]

Done — agent-storage.ts was split into a 43-line barrel + 8 mixin modules (base/activity/vectors/collections/files/knowledge/summaries/context) in c77a46b.