setup_builder: take the buildkitd GC keepDuration from the sticky disk response

[piob-io]

Summary

Lets the backend control buildkitd's GC TTL (today hardcoded keepDuration = "192h" in DEFAULT_BUILDKITD_CONFIG) per organization and fleet-wide, without an action release. No new request: the agent already resolves the sticky disk through the backend on GetStickyDisk, and that response now carries buildkitdConfig { gcKeepDurationHours } (FastActions/fa#5384, sourced from useblacksmith/web#12355).

getStickyDisk()            → buildkitd_config: buildkitdConfigFromServer(response.buildkitdConfig?.gcKeepDurationHours)
setupStickyDisk()          → { device, exposeId, buildkitdConfig }
startAndConfigureBuildkitd(parallelism, buildkitdPath, driverOpts, serverConfig?)
startBuildkitd(..., serverConfig?) → writeBuildkitdTomlFile(..., serverConfig)   // existing plumbing, now fed

buildkitdConfigFromServer(hours) only overrides keepDuration (72 → "72h"); gc = true, all = true, parallelism and everything else stay as before. Missing/0 (old agent or backend, non-dockerfile disks) → default silently; non-integer or outside 1..8760 → core.warning + default. v1 (main) does not read the field and is unchanged.

Note on types: @buf/blacksmith_vm-agent.bufbuild_es is pinned to a BSR build that predates the new proto field, so the field is read via the same structural cast the function already uses for exposeId etc. Until fa#5384 merges and pnpm update-proto bumps the package, protobuf-es leaves field 5 in $unknown and the action keeps using 192h — i.e. this PR is safe to ship first; the bump is what activates it.

Link to Devin session: https://app.devin.ai/sessions/5c8deaae10f14cf49877e6f03ff873f9
Open in Devin Desktop: https://app.devin.ai/desktop/session/5c8deaae10f14cf49877e6f03ff873f9?variant=devin
Requested by: @piob-io

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