feat: add posthog log drain adapter

[HugoRCD]

This pull request introduces first-class support for PostHog as a log drain adapter in the evlog observability framework. It adds the PostHog adapter implementation, documentation, and integration points throughout the codebase and docs. This enables users to send server-side logs and wide events to PostHog for product analytics, session replay correlation, and advanced querying.

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Project	Deployment	Actions	Updated (UTC)
evlog-docs	Ready	Preview, Comment	Feb 7, 2026 5:47pm

[github-actions]

Thank you for following the naming conventions! 🙏

[Copilot]

Pull request overview

This PR adds a new PostHog log drain adapter to evlog, along with supporting package exports, tests, and documentation so users can route wide events into PostHog via the evlog:drain hook.

Changes:

• Added a PostHog adapter (createPostHogDrain, sendToPostHog, sendBatchToPostHog, toPostHogEvent) with runtime/env configuration support.
• Exposed the adapter as evlog/posthog via build + package export wiring, and added a comprehensive Vitest suite.
• Extended docs and the playground app to include PostHog adapter guidance and a drain test endpoint/UI entry.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/evlog/test/adapters/posthog.test.ts	Adds unit tests for PostHog event mapping and HTTP batching/timeout behavior.
packages/evlog/src/adapters/posthog.ts	Implements the PostHog drain adapter and PostHog Batch API sender.
packages/evlog/package.json	Exposes evlog/posthog via exports and typesVersions.
packages/evlog/build.config.ts	Adds the PostHog adapter entrypoint to the build outputs.
apps/playground/server/plugins/evlog-drain.ts	Adds commented example wiring for the PostHog drain in the playground.
apps/playground/server/api/test/drain.get.ts	Adds an endpoint to emit a wide event intended to exercise drain adapters.
apps/playground/app/config/tests.config.ts	Adds a “Drains” test card to trigger the new drain test endpoint.
apps/docs/content/3.adapters/5.custom.md	Links the new PostHog adapter from the custom adapters page.
apps/docs/content/3.adapters/4.posthog.md	New documentation page describing installation/configuration/usage for PostHog.
apps/docs/content/3.adapters/3.otlp.md	Adds “Next steps” link to the PostHog adapter docs.
apps/docs/content/3.adapters/2.axiom.md	Adds “Next steps” link to the PostHog adapter docs.
apps/docs/content/3.adapters/1.overview.md	Adds PostHog to the adapters overview page and env var examples.
AGENTS.md	Documents the new adapter entrypoint and minimal setup instructions.

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