feat: embedded fill Dockerfile + state-actor/eest build-run CI

[skylenet]

Follow-up to #248. Two related changes that, together, let CI exercise the full state-actor → eest_payloads → run pipeline on a GitHub-hosted runner.

1. Build the fill image from an embedded Dockerfile by default

builder.eest_payloads.fill_image and fill_dockerfile are now both optional. When neither is set, benchmarkoor builds the fill image from a Dockerfile.eest-filler embedded in the binary (go:embed). The Dockerfile copies nothing from the build context (execution-specs is bind-mounted at run time), so it builds against an empty temp dir written by resolveFillDockerfile and cleaned up afterwards.

• Moved Dockerfile.eest-filler into pkg/builder/ so it can be embedded (.dockerignore doesn't exclude it, so the in-image Go build still embeds it).
• BuildsFillImage() now also builds when no fill_image is configured; dropped the "one of fill_image/fill_dockerfile is required" validation.
• Dropped the now-unnecessary fill_dockerfile line from the example configs; updated docs + config.example.yaml.
• Tests: TestEmbeddedFillDockerfile, TestResolveFillDockerfile (configured + embedded paths, with cleanup), TestBuildsFillImage truth table.

2. state-actor + eest build/run e2e via the composite action

• action.yaml — optional build phase (build-config / build-config-urls / build-args) that runs benchmarkoor build before the existing run step. Existing jobs are unaffected (the phase is skipped when no build config is passed).
• ci.action.yaml — new state-actor job: builds OSAKA nethermind + besu snapshots, fills the compute bn128 subset with the besu filler, then replays the fixtures on nethermind and besu. datadir_method: copy + a 256 MB snapshot keep it within an ubuntu-latest runner. Runs on PRs (consistent with the other action jobs).

Validation

• Full go test ./... green (except the pre-existing macOS-only /proc/mounts schelk tests); golangci-lint --new-from-rev=origin/master 0 issues; actionlint clean.
• VM end-to-end with the embedded Dockerfile and no fill_dockerfile: fill image built from the embed → 36/36 filled (osaka bn128), nethermind + besu snapshots OK, replay 72/72 (36 each), 0 failed — including verifying cleanup_on_start does not wipe the freshly-built snapshots.