test: verify PostHog timestamps use UTC

[marandaneto]

Problem

The plugin relies on posthog-node to add timestamps when it sends events. We did not have regression coverage proving that these timestamps stay in UTC when OpenCode runs in a non-UTC timezone.

Changes

This adds an integration test that runs the plugin with TZ=America/Los_Angeles and freezes the clock at July 1, 2024 at 12:34:56.789 local time. The test sends an assistant message and a completed generation through the real plugin path, captures the compressed request with a local HTTP server, and verifies both sent_at and the $ai_generation timestamp are exactly 2024-07-01T19:34:56.789Z.

How did you test this code?

This change was prepared by an agent. I ran these automated checks:

• pnpm test - 48 tests passed across 4 test files.
• pnpm lint - oxlint and oxfmt checks passed.
• pnpm typecheck - TypeScript type checking passed.

Checklist

• Tests for new code (if applicable)
• Accounted for backwards compatibility of any changes (no breaking changes!)

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

A Pi implementation agent prepared the test from a human-directed request. The test uses the real posthog-node request path rather than mocking timestamp generation so it covers the serialized payload that PostHog receives. Pi autoreview checked the committed branch against origin/main and reported no actionable findings.

[arnohillen]

(created by claude code)

[arno's agent] Verified at 27b5451: test-only, drives the real plugin path through a local HTTP server and asserts the gzip batch carries the canonical UTC instant.

[arnohillen]

(created by claude code)

[arno's agent] The PR body still mentions a changeset, but 27b5451 removed it and the diff is now test-only, so that sentence is stale.

[marandaneto]

Fixed — I removed the stale changeset references from the PR description now that the diff is test-only. Thanks for catching that!