ci: gate the Go collector MVP path

[zzylol]

Summary

• test the repository-owned Go precompute and Gorilla contracts
• test the asapedgeprocessor CollectorPlan/OpAMP runtime boundary
• test the Python MVP harness contracts and validate the demo entrypoint
• initialize the pinned Collector core and Go sketch dependencies required by those tests
• rerun the gate when a stacked PR changes its base

Why

The existing MVP collector job only exercised asap-precompute-rs. The documented Collector MVP runtime is Go, while Rust is one precompute/contract implementation. This change makes the required check cover both sides without claiming that pre-merge tests replace the canonical four-node measured run.

Verification

• workflow YAML parses successfully
• git diff --check
• GitHub Actions is the authoritative clean-checkout run for pinned submodules and private dependencies

MVP evidence boundary

This gate proves code-level contracts and harness behavior. The canonical acceptance evidence remains deploy/mvp-multinode/scripts/run_demo.sh all, which must produce MVP_RESULTS.json and MVP_REPORT.md.

[zzylol]

The Go gate is intentionally red and has exposed a real clean-checkout blocker, not a CI-only issue:

• asap-precompute-go consumes GOS APIs split across open sketchlib-go PRs feat: benchmark delta transmission — accuracy, payload size, CPU/memory (#66) #69, ASAPQuery Integration #74, KLL: OTLP export and sketch serialization #75, CountMinSketch: OTLP export and sketch serialization #76, and CountSketch: OTLP export and sketch serialization #77.
• The version recorded in asap-precompute-go/go.mod predates those APIs.
• No single committed sketchlib-go revision currently contains the full API set, so pinning one feature head cannot make the Collector Go runtime reproducible.

This PR should not be weakened back to Rust-only coverage. The dependency stack needs to land (and ASAPCollector needs one consolidated pin) before this required gate can become green.