Add authorization visibility and retention policy for context memory

[michaelmwu]

Parent: #265

Goal

Add deterministic authorization, visibility, consent, and retention policy for context and memory.

Scope

• Define context/memory scopes:
  • context:read_current_thread
  • context:read_channel_recent
  • context:read_user_recent_self
  • context:read_user_recent_any
  • memory:read_self
  • memory:read_project
  • memory:write_self
  • memory:write_project
  • memory:admin
• Enforce source visibility:
  • whether the actor can see the original source
  • whether the source can be used for the current purpose
  • whether the response destination can receive the result
• Define retention defaults for thread snippets, summaries, and durable facts.
• Require confirmation or admin policy for writes to durable memory.

Acceptance criteria

• Context retrieval is denied when the actor cannot see the source.
• Private memory is not echoed into public destinations.
• Durable fact writes require explicit confirmation or admin authority.
• Retention/expiry rules are represented in code and tests.
• Policy decisions are auditable.