Session persistence and conversation replay on load

[bug-ops]

Parent Epic

#778

Problem

No session persistence. load_session only checks in-memory map. IDE cannot resume a previous conversation after agent restart.

Solution

• Persist conversation history to SQLite (reuse zeph-memory infrastructure)
• Store per-session: messages, tool calls/results, metadata (model, timestamps)
• load_session loads from DB and replays as session/update notifications:
  • Text messages → AgentMessageChunk
  • Tool calls → ToolCall (status: Completed/Failed)
  • Tool results → ToolCallUpdate
  • Thinking → AgentThoughtChunk
• Only replay user_visible messages (skip internal system prompts)

Critical Path

Depends on: Epic 1 (#777) complete
Parallel with: #781 (multi-session)
Blocks: Epics 3-8

Acceptance Criteria

• Conversation stored to SQLite after each turn
• load_session replays full history as notifications
• Session survives agent restart
• Integration test: create session → prompt → restart → load → verify replay