Session end lifecycle missing for OpenCode, Gemini CLI, and Codex

[Alan-TheGentleman]

Context

PR #47 fixes session-stop for Claude Code by calling POST /sessions/{id}/end from the hook. But the other three agents have no equivalent — their sessions always have ended_at = NULL.

Current state

Agent	Session start	Session end
Claude Code	session-start.sh hook → POST /sessions	session-stop.sh hook → POST /sessions/{id}/end (PR #47)
OpenCode	engram.ts plugin → POST /sessions	Nothing
Gemini CLI	MCP mem_session_start via protocol	Nothing
Codex	MCP mem_session_start via protocol	Nothing

What's already available

The MCP tool mem_session_end exists and works. Agents just need to be told to call it.

Proposed fix per agent

• OpenCode: Add a session-end event handler in engram.ts that calls POST /sessions/{id}/end (same pattern as the session creation handler)
• Gemini CLI: Add instruction in the system prompt telling the agent to call mem_session_end before closing
• Codex: Add instruction in the memory config telling the agent to call mem_session_end before closing

Why it matters

Without ended_at, session duration is unknown, the session list grows unbounded with no way to tell which sessions are active vs finished, and any future cleanup/retention logic can't distinguish stale sessions from ongoing ones.