chore(devex) evaluate CI platform alternatives

Context

We currently use GitHub Actions with some runners hosted on Depot. Depot has since launched a dedicated CI product, and there are other alternatives worth exploring (GitLab Runners, self-hosted infrastructure). Worth doing a structured evaluation before the next renewal cycle.

Goal

Determine whether an alternative CI platform or runner setup could reduce cost, improve speed, or both — without sacrificing coverage or developer experience.

Scope / ideas

• Depot CI — we already use Depot for runners; evaluate their full CI product for tighter integration and potential cost savings
• GitLab Runners — evaluate as a runner backend (can be used with GitHub via mirroring or as a full migration path)
• Self-hosted runners — cost model analysis for hosting our own (e.g. on spare capacity, spot instances)
• Hybrid approach — keep GitHub Actions as orchestrator but swap runner backends
• Compare on: cost, cold-start latency, caching, maintenance burden, and migration effort

Related to https://github.com/PostHog/posthog/issues/51012 and https://github.com/PostHog/posthog/issues/51027 but a bit more big picture (if we want to go that route)

Start by reviewing related issues #51012 and #51027, then document the options named here: Depot CI, GitLab Runners, self-hosted runners, and a hybrid setup. Compare cost, cold-start latency, caching, maintenance burden, coverage, developer experience, and migration effort; done means producing a structured evaluation before the next renewal cycle.