[pi-extension] Turn Storage & Multi-Agent Support

[codenamekt]

Overview

Improve turn storage and support multi-agent memory sharing.

Issues to Address

• Turns stored unbounded — add TTL per session (configurable, default 30 days) + background session pruning job
• Turns not associated with project — allow project_id on append_turn for scoped recall
• No turn summarization — add hexus_summarize-turns tool to condense old turns into a single memory entry
• Agent identity is single-namespace — support hexus_recall with agent_identity: ["pi", "hermes"] for cross-agent memory search

API Changes Needed

# append_turn additions
POST /api/append-turn
{
  "session_id": "...",\n  "role": "assistant",\n  "content": "...",\n  "agent_identity": "pi",        # existing
  "project_id": "project-x",    # NEW
  "ttl_days": 30                 # NEW, optional override\n}

# recall additions
POST /api/recall
{
  "query": "...",\n  "top_k": 5,
  "agent_identity": ["pi", "hermes"],  # NEW: can be list for cross-agent
  "project_id": "project-x"             # NEW: scope to project
}

Acceptance Criteria

• Turns auto-expire after TTL (30 days default)
• Pruning job endpoint: POST /api/prune-sessions
• hexus_summarize-turns tool condenses session turns into a memory entry
• agent_identity accepts array, returns memories from all specified agents
• project_id scopes recall and storage to a project