Let invoke_planner start an automatic follow-up behind a system notice

[lavaman131]

An attached Planner client or an MCP caller sometimes needs a Planner turn that nobody typed, such as "a workflow run finished; update the document". Until now invoke_planner could only post the instruction as the caller's member message.

What changes

invoke_planner takes an optional as: "system":

{
	"id": "/documents/octo-org/score/release-readiness",
	"instruction": "The workflow run finished. Read its results and update the document.",
	"as": "system"
}

• The turn goes through the existing instruct() path. Chat shows the system notice Automatic follow-up requested by @login. instead of a member message. The instruction reaches the Planner alone, and it stays out of the visible queue while it waits.
• Only the document's creator, or the account whose client holds its Planner, may send one. Anyone else gets system-turn-forbidden. For a child document, the check uses its root.
• Each document accepts at most 10 per hour, counted in memory. Past that, the call gets system-turn-limited. A follow-up that is refused later (for example planner-not-attached) does not use up the limit.
• Every accepted or refused follow-up is logged.
• Ordinary invoke_planner calls and every other refusal behave as before. Nothing changes in the agent-connection protocol: an attached client calls invoke_planner with a GitHub token of the account that holds the Planner.

Docs: local-agent-mcp.md ("Automatic follow-ups"), hosted-agent.md, agent-connection.md, authentication.md, AGENTS.md.

Verification

• New unit tests in apps/server/src/mcp/invoke.test.ts, written to fail first, cover:
  • a system turn starts with the instruction and shows only the notice
  • a queued system turn stays out of the queue frames
  • non-owners are refused, and the Planner's holder is allowed
  • the hourly limit, and that ordinary calls don't count against it
  • argument validation and the new refusal codes
• hosted.test.ts checks that as: "system" reaches the invocation.
• bun test apps/server packages apps/web: 5790 pass, 0 fail. bun run types and bun run ci pass.
• Browser: mcp-update, planner-attachment, and chat-mentions e2e pass (13 tests).