fix(genui): stop GenUI reminder from triggering an extra turn - #56548

Why

In the OpenCode app, sending a first prompt with Generated UI enabled (metadata.genui = true) caused the assistant to reply twice to a single prompt. In session ses_eb4805506ffdExxfN3euuGJzSR, the user sent "hi" at 1792268020571, assistant 1 completed at 1792268021922, a synthetic message containing the GenUI <system-reminder> was delivered at 1792268021930, and assistant 2 started at 1792268021939 and greeted the user a second time.

GenUISession's context hook both spliced Message.user(enter) into event.messages and called ctx.session.synthetic({ sessionID, text, resume: false }). Because Session.synthetic admits a "steer" inbox item and resume: false only skips execution.wake(), the already-active runner promoted the synthetic item in advanceToStep after step 1 finished, running an extra model step and persisting the reminder after the assistant's reply (diverging from the order the model saw).

What Changes

Make the GenUISession context hook side-effect free by providing GenUI guidance as a system part (event.system.push(SystemPart.make(...))), matching packages/core/src/tool/plugin/opencode.ts:

• Before: On a context hook call, GenUISession scanned event.messages for enter/leave <system-reminder> user messages, spliced a user message into event.messages, and called ctx.session.synthetic({ sessionID, text, resume: false }), queuing a steered synthetic message while a step was already running.
• After: The prompt hook records whether the latest prompt had metadata.genui === true (and session.deleted cleans up the map entry). On context, if GenUI is offered for the session, GenUISession pushes SystemPart.make(instructions) onto event.system and keeps genui_render; otherwise it deletes genui_render from event.tools and adds nothing. It never mutates event.messages or enqueues synthetic inbox items.

sequenceDiagram
  participant A as App
  participant R as Runner
  participant H as GenUISession (context hook)
  participant M as Model

  Note over A,M: Before
  A->>R: prompt ("hi", metadata.genui = true)
  R->>H: context hook
  H->>R: splice Message.user(enter) + session.synthetic(resume: false)
  R->>M: step 1 (user: enter, user: "hi")
  M-->>R: greeting 1
  R->>R: advanceToStep promotes steered synthetic item
  R->>M: step 2 (persisted synthetic reminder)
  M-->>A: greeting 2

  Note over A,M: After
  A->>R: prompt ("hi", metadata.genui = true)
  R->>H: context hook
  H->>R: event.system.push(SystemPart.make(instructions))
  R->>M: step 1 (system includes GenUI guidance, user: "hi")
  M-->>A: greeting 1

Scope

This PR keeps the fix small in packages/plugin-genui/src/session.ts using existing event.system support on the context hook, plus unit and runner regression tests. A separate follow-up PR will move GenUI onto keyed session instruction entries once plugins can access them.

Verification

# packages/plugin-genui
taskpolicy -c utility bun test test/session.test.ts
skulk run --slot heavy --stable 0s -- taskpolicy -c utility bun run typecheck

# packages/core
taskpolicy -c utility bun run script/test.ts test/session-runner.test.ts -t "runs one model request with system guidance when a prompt enables GenUI"
skulk run --slot heavy --stable 0s -- taskpolicy -c utility bun run typecheck

• Targeted GenUI session tests: 4 pass, 0 fail. Cover enabled guidance on repeated context calls, disabled and missing metadata, and unknown sessions; messages remain unchanged and no synthetics are enqueued.
• Runner regression: 1 pass, 0 fail. Uses the real tool registry and verifies exactly one model request, with both genui_render and its system guidance, only "hi" in user input, and no persisted reminder.
• Both affected package typechecks and formatting checks pass.
• Repository commit/push hooks also passed their checks, including the full typecheck graph; existing lint warnings remain.