fix(build-summary): mount schelk before rendering so a skipped state-actor shows its data

[skylenet]

Symptom

In the GitHub Action's build summary, a skipped state-actor target shows only Status: Skipped, Client, and Elapsed — no fork, state root, docker image, account/contract/storage counts, or DB size. The eest-payloads section on the same run shows full data.

Root cause

The build markdown summary is rendered by a separate generate-build-markdown-summary step that reads each target's on-disk output_dir (state-actor state-actor-manifest.json, eest .benchmarkoor-fill.json). The state-actor output_dir lives under a schelk mount (/schelk/state-actor/v1/nethermind); a promoted state-actor datadir is left unmounted (schelk promote). So when the render step runs after the build, the manifest reads as missing → the renderer shows nothing beyond the fields already in build-summary.json (status/client/elapsed).

The eest section is fine because its fill sidecar isn't under schelk.

Fix

generate-build-markdown-summary now ensures the schelk scratch is mounted before rendering, when any target's output_dir is under a schelk mount:

• New exported executor.ReadBuildSummary(path) (also used by GenerateBuildMarkdown now).
• The cmd reads the summary, and if any output_dir is schelk-backed, calls datadir.EnsureSchelkMounted once (one volume covers all). Best-effort — a non-schelk host or unreadable summary is a no-op; a mount failure logs a warning and renders what it can (no regression).

The render step already runs with sudo -E in the action, so it can mount.

The executor stays free of a datadir dependency — the schelk-ensure lives in the cmd layer.

Tests / validation

• Existing executor + cmd tests pass (GenerateBuildMarkdown refactored onto ReadBuildSummary); go build/go test green; golangci-lint --new-from-rev=origin/master 0 issues.
• The live schelk path can't be exercised here (needs a schelk-initialised Linux host); worth confirming on a benchmarkoor-tests re-run.