fix(session): skip synthetic delegation nudge in subagent sessions

[vicary]

Issue for this PR

Closes #17202

Type of change

• Bug fix

What does this PR do?

When a user message contains an @agent mention, SessionPrompt.createUserMessage injects a synthetic text part instructing the model to call the task tool with that subagent. This is correct behaviour in a root session where the user is initiating a delegation.

In a child/subagent session however, the orchestrator has already written an explicit task prompt before the session starts. When the subagent processes its own message history it encounters the @agent part and the injection runs again, appending a second delegation instruction. This causes recursive nesting bias: the subagent reads its own context, sees the nudge, and attempts to spawn yet another delegation layer.

The fix is a minimal guard in the if (part.type === "agent") branch of createUserMessage:

const currentSession = await Session.get(input.sessionID)
if (currentSession.parentID) {
  return [agentPart]
}

Session.Info.parentID is already set by the orchestrator when it creates child sessions. The guard is minimal and does not affect any other code path.

How did you verify your code works?

• Read the injection logic in packages/opencode/src/session/prompt.ts to confirm the root cause.
• Confirmed Session.Info.parentID is populated for child sessions via packages/opencode/src/session/index.ts.
• Built a patched binary locally (OPENCODE_VERSION=1.2.24 bun run ./packages/opencode/script/build.ts).
• Ran a multi-agent session end-to-end: root session delegated to a subagent via @agent. Verified via DB inspection that the subagent's message history no longer contains the synthetic delegation text part.
• Verified root-session delegation still works correctly (synthetic text still injected when parentID is absent).
• Added two unit tests to packages/opencode/test/session/prompt.test.ts:
  • injects synthetic delegation nudge in root sessions
  • skips synthetic delegation nudge in subagent sessions
• All 6 tests in prompt.test.ts pass (bun test ./test/session/prompt.test.ts).

Screenshots / recordings

No UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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