Determinize release: hardcode OptProf baseline + Phase 3.2 baseline resolver

[JanProvaznik]

Two release-process determinizations, plus re-includes the Phase 3.2 baseline helper that missed the #14220 merge.

1. Hardcode the OptProf baseline (new)

A freshly-cut vs* branch has no OptProf optimization data of its own, so its first official build fails — today handled by the manual Phase 4.4 dance (cancel the auto-build, re-run with an Optional OptProfDrop Override). This makes it automatic:

• .vsts-dotnet.yml: new OptProfBaselineDrop variable holding the latest known-good main OptimizationData drop. On vs* branches the build seeds OptProfDrop from it (→ /p:VisualStudioIbcDrop) and clears SourceBranch, so the first build of a brand-new branch has valid OptProf data.
  • main is unaffected (still resolves its own OptProf via SourceBranch); the manual OptProfDropName override still wins; existing vs* branches keep their frozen copies (forward-only change).
• scripts/Get-LatestOptProfDrop.ps1: resolves the value deterministically from the MSBuild-OptProf pipeline (def 17389) latest successful main run, reading the Set PreviousOptimizationInputsDropName step. Validated → OptimizationData/DotNet-msbuild-Trusted/main/20260623.5/14471019/1.
• Process: new checklist Phase 3.3 refreshes OptProfBaselineDrop in the main-bump PR (API-compat → 3.4, merge → 3.5). Phase 4.4 demoted to a fallback (only if the seeded build still fails).

⚠️ Infra change to the official build — worth a careful review of the ${{ if startsWith(... 'refs/heads/vs') }} seeding block and a validation run on a vs* branch. Trade-off: a vs* branch now uses the OptProf data pinned at cut time (deterministic; fine for a servicing branch) rather than resolving its own over its lifetime.

2. Deterministic Phase 3.2 baseline (re-include)

scripts/Get-PackageValidationBaseline.ps1 + its doc wiring — this was committed after #14220 had already merged at its first commit, so it never landed. Re-included here. Resolves PackageValidationBaselineVersion via git merge-base → pipeline 9434 → dotnet-tools feed. Validated → 18.9.0-preview-26330-01.

Both scripts require az login with devdiv access; the manual procedures remain as fallbacks.

[github-actions]

🔍 Skill Validator Results

⚠️ Warnings or advisories found

Scope	Checked
Skills	1
Agents	0
Total	1
Severity	Count
---	---:
❌ Errors	0
⚠️ Warnings	1
ℹ️ Advisories	0

Summary

Level	Finding
ℹ️	Found 1 skill(s)
ℹ️	[release] 📊 release: 3,668 BPE tokens [chars/4: 3,588] (standard ~), 13 sections, 1 code blocks
ℹ️	[release] ⚠ Skill is 3,668 BPE tokens (chars/4 estimate: 3,588) — approaching "comprehensive" range where gains diminish.
ℹ️	✅ All checks passed (1 skill(s))

Full validator output

```text Found 1 skill(s) [release] 📊 release: 3,668 BPE tokens [chars/4: 3,588] (standard ~), 13 sections, 1 code blocks [release] ⚠ Skill is 3,668 BPE tokens (chars/4 estimate: 3,588) — approaching "comprehensive" range where gains diminish. ✅ All checks passed (1 skill(s)) ```

[Copilot]

Pull request overview

This PR determinizes two parts of the MSBuild release process: (1) ensuring newly-cut vs* branches have a known-good OptProf baseline so the first official build succeeds without manual intervention, and (2) reintroducing a deterministic resolver script for PackageValidationBaselineVersion (Phase 3.2) and wiring it into the release checklist + release skill docs.

Changes:

• Add a hardcoded OptProfBaselineDrop in .vsts-dotnet.yml and seed OptProfDrop from it on vs* branches (while keeping main behavior unchanged).
• Add/re-include two helper scripts to deterministically resolve (a) the latest OptProf drop and (b) the package validation baseline.
• Update release checklist and release skill documentation to incorporate the new deterministic steps and revised Phase numbering.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
scripts/Get-PackageValidationBaseline.ps1	New helper script to compute PackageValidationBaselineVersion via merge-base + pipeline query + dotnet-tools feed verification.
scripts/Get-LatestOptProfDrop.ps1	New helper script to pull the latest OptProf drop from MSBuild-OptProf pipeline logs.
documentation/release-checklist.md	Updates Phase 3 to reference the new scripts and documents the new OptProf baseline refresh + fallback behavior.
.vsts-dotnet.yml	Adds OptProfBaselineDrop and seeds OptProfDrop for vs* branches to avoid first-build failures.
.github/skills/release/SKILL.md	Documents the deterministic baseline resolver and adds OptProf baseline refresh into Phase 3 guidance and quick links.