feat(mcp): session/reader registry + resource

[radimsem]

Type: AFK

What to build

Only an implicit len(Sessions()) count exists today. Add a lightweight concurrency-safe registry of active MCP client sessions (id, transport stdio|http, connected-at, last-activity) and the db path the server is bound to. A read-only resource exposes it so the desktop client can show "who's attached to this brain."

remindb://sessions →
{ "db_path": "",
  "sessions": [ { "id": "", "transport": "stdio|http",
                  "connected_at": 0, "last_activity": 0 } ] }

Acceptance criteria

• Server maintains a concurrency-safe session registry, updated on connect / disconnect / activity
• Resource lists + reads the registry JSON
• Registry count agrees with the MCP SDK's session set
• Follows the feat(mcp): resource infrastructure + overview resource #104 resource contract (passive: no boost, no OpMu, no snapshot)
• skills/remind/SKILL.md updated
• Test asserts a session appears/disappears across connect/disconnect
• make test green

Blocked by

• feat(mcp): resource infrastructure + overview resource #104