agent-context-graph: MCP server with the recall tool

[antejavor]

Part of #390. Implements #394 points 1, 2, 4 and 9.

What

• agent-context-graph mcp: a stdio MCP server, with the mcp SDK as an optional extra. Identity and config resolve from config.toml only.
• Tools: contributed through a new agent_context_graph.tools entry-point group. sessions-graph registers recall(question), which returns the rendered text plus the same data as structured content.
• CLI: recall prints the text, or the JSON with --json.
• SessionStart: adds one line of context pointing the model to recall. No retrieved content is pushed.
• Plugins: the Claude Code and Codex plugins bundle the MCP server.

Done when

• recall works end to end in Claude Code against a real session graph.
• Checked and written down:
  • what Claude Code and Codex show the model when a result has both text and structured content;
  • whether Codex plugins can bundle an MCP server;
  • that SessionStart context injection works in both.