chore(ci): move turbo caching to actions/cache via shared setup action

[joshuaellis]

Why

The Turbo remote cache has been silently dead — TURBO_TOKEN fails authentication on every run, so CI rebuilds everything cold (0 cached, 8 total) and the logs are littered with auth warnings.

What

Switch to GitHub's native actions/cache for Turbo's .turbo directory, so build output is reused across runs with no Vercel token required. A shared setup composite action absorbs the pnpm + node + install boilerplate every workflow was hand-rolling, with an opt-in flag for the cache.

Alongside this, the Tests workflow stops doing work it never needed: unit tests run against source, so the redundant build step is gone, and the unit/type/e2e jobs no longer serialise behind a build job whose artifacts they never consumed. The Turbo build input was also still pointing at the removed tsup config — it now tracks tsdown, so bundler-config changes correctly invalidate the cache.

[changeset-bot]

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Latest commit: 70171bc

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