ci: export a minimal cache for API builds

[Medformatik]

What changed and why

API builds now export a minimal cache (mode=min) in PR validation and trusted image builds. Other image targets keep max.

In the two-replicate comparison, cold export median fell from 344s to 59s. Mean worker time per cold → unchanged → source-only chain fell from 685.5s to 440s. These controlled measurements do not establish an overall CI speedup. The developer guide records the results, trade-offs and follow-up measurements needed.

Remove the temporary benchmark workflow, composite action, collector and tests from #496 now that the comparison is complete. Keep a concise record of the results and cache policy in the developer guide.

Related work

Continues #496 and the approved repository-readiness CI work.

Verification

Checks recorded during the original PR:

• Qualification: nine builds passed; repeated comparison: all 18 builds and report passed at the same revision/toolchain.
• 97 focused CI/release tests and infrastructure validation passed after removing the benchmark.
• Docs production build, typecheck and formatting passed; no references to the removed tooling remain.
• pnpm verify:full passed: lint, workspace types, policy, OpenAPI and 19,693 tests (186 expected skips).

Release and upgrade impact

Only API Docker builds switch to minimal cache export; other image targets retain max. Temporary #496 tooling is removed.