feat(builder): auto-mount + promote schelk datadirs on state-actor build

[skylenet]

When a builder.state_actor target's output_dir lives under a schelk mount, benchmarkoor build now integrates with schelk so the built datadir becomes the runner's baseline automatically.

What

1. Ensure mounted — before inspecting or writing the datadir, mount the schelk scratch (schelk mount -y if /proc/mounts shows it unmounted; hard error on the mounted-but-not-in-/proc/mounts crash case, pointing at schelk full-recover). This matters because the datadir is materialised onto the mount and the skip/diff checks must read the mounted content, not an empty mount point.
2. Promote — after a build that actually ran, schelk promote -y persists the new datadir as the virgin baseline. A skipped (unchanged) target is not promoted; with the diff-detection work, "a build ran" is exactly "the datadir changed". A failed promote is fatal — leaving the mount ahead of the baseline would let a later schelk recover silently revert to the stale datadir.

Detection is automatic from the output_dir path (via /var/lib/schelk/state.json); when schelk isn't set up, non-schelk builds are completely untouched.

Changes

• pkg/datadir/schelk.go: SchelkDir (is output_dir under the schelk mount? absent state → not schelk), EnsureSchelkMounted (the mount/crash-inconsistent logic), SchelkPromote.
• pkg/config/config.go: refactored the runner's schelk mount-preflight to reuse EnsureSchelkMounted — removes the duplicated switch while preserving its full-recover error and the schelk mount -y call.
• pkg/builder/state_actor.go: ensure-mount hook before the skip checks; promote hook after the sidecar write.
• Tests + a docs note under the schelk section.

Validation

• go build ./... clean; golangci-lint --new-from-rev 0 issues; new datadir tests pass (incl. a fake-schelk shim asserting promote -y is invoked and failure output is surfaced).
• The config schelk mount-preflight test passes on Linux (VM), confirming the shared-helper refactor is behavior-equivalent (the macOS failure is the pre-existing /proc/mounts limitation — EnsureSchelkMounted reads /proc/mounts exactly as the old inline code did).

Not covered

The live mount+promote path was not e2e'd — it needs a schelk-initialized Linux host (block devices, dm-era, root), which wasn't available. It's covered by unit tests + the Linux config test + code review, but a manual smoke test on a schelk-enabled box is worthwhile. Note schelk ops require root, so benchmarkoor build must run as root for schelk output_dirs (consistent with the runner; it fails loudly otherwise).