Persist task history behind MemoryStore interface

[David-patrick-chuks-02]

Description

Task history currently lives in memory. Add an optional persistent MemoryStore implementation so contributors can experiment with durable task history.

Acceptance Criteria

• File or JSON-backed store implements existing MemoryStore interface
• Runtime can select in-memory (default) or persistent store via config
• Tests cover write/read/list of task history
• Default behavior remains in-memory

Technical Notes

• Do not introduce heavy external DB dependencies in this issue
• Keep interface-first design intact

Labels

enhancement help wanted