feat(agent): forward ai_stage as a gateway trace property

[joshsny]

Problem

The signals agentic research runs (repo selection + report research) execute inside the sandbox agent, whose LLM calls route through the gateway tagged ai_product=signals. They carried signal_report_id (for the research run) but no ai_stage, so the spend couldn't be broken down by pipeline stage the way the rest of the signals pipeline can.

Changes

configureEnvironment now reads ai_stage from the task run state (getTaskRunStateString(preTaskRun, "ai_stage")) and includes it in the buildGatewayPropertyHeaders bag, so it's forwarded as an x-posthog-property-ai_stage header on the agent's Anthropic calls. The gateway lifts it onto the $ai_generation event.

The producing side — setting TaskRun.state["ai_stage"] to "research" / "repo_selection" and extending signal_report_id to the repo-selection run — is in the companion PostHog/posthog PR (#64367).

How did you test this?

I'm an agent (PostHog Slack app). I ran locally:

• vitest run on agent-server.configure-environment.test.ts — 16 passed, including a new assertion that ai_stage is forwarded and one that it's omitted when absent.
• tsc --noEmit typecheck on @posthog/agent — clean.
• biome check on the changed files — clean.

Automatic notifications

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 7eb5cf2.}

[greptile-apps]

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