Pin spawned Claude Code CLI to compiled-in feature-gate defaults

[jhampton]

Problem

The spawned Claude Code CLI resolves feature gates through a remote GrowthBook config. One of those gates (tengu_amber_heron, compiled default off) controls whether a Task subagent launches synchronously (blocking until the child finishes) or asynchronously (fire-and-forget).

The launch decision reduces to:

isAsync = isRemote
  || run_in_background === true
  || agentDef.background === true
  || teamsMode || tasksMode
  || (!isTeammate && run_in_background !== false && featureGate("tengu_amber_heron", false))

When that remote gate flips on, subagents launch async by default even when run_in_background was never set. Async launches live inside the SDK subprocess and are torn down when it is disposed/recreated, and callers that only track explicitly backgrounded tasks never see them. The net effect: spawned-CLI behavior can change mid-flight — between two runs of the same code, same CLI version — purely because a remote flag was toggled. That is a reproducibility hazard for any host that relies on blocking subagent semantics.

Fix

Set DISABLE_GROWTHBOOK in buildClaudeSubprocessEnv() so the spawned CLI resolves every feature gate to its compiled-in default. Behavior becomes deterministic and independent of remote config; the async-launch gate stays at its compiled default (off) and cannot silently flip.

Notes:

• Explicit run_in_background: true still works — this only pins the default. It is deliberately narrower than CLAUDE_CODE_DISABLE_BACKGROUND_TASKS, which removes the run_in_background parameter from the tool schemas entirely.
• An existing DISABLE_GROWTHBOOK value (from the environment or envOverrides) is respected, so operators can opt back into remote gates.
• More broadly, this removes remote-config influence over spawned-CLI behavior — the subprocess behaves the same offline as online.

Test plan

Adds packages/shared/tests/claude-subprocess-feature-gates.test.ts:

• pins DISABLE_GROWTHBOOK=1 so feature gates resolve to compiled defaults.
• respects a DISABLE_GROWTHBOOK value already present in the environment.
• respects a DISABLE_GROWTHBOOK value passed via envOverrides.

Validation:

• bun test tests/claude-subprocess-feature-gates.test.ts in packages/shared → 3 pass, 0 fail.
• tsc --noEmit on packages/shared → 0 errors.