Add MSBuild quality review agentic workflow (#19958)

Port and adapt microsoft/testfx PR #8365 ("MSBuild quality review") to a
gh-aw workflow for dotnet/fsharp. Runs weekly, audits the repo's .props /
.targets / .Targets files for MSBuild authoring issues, files an issue with
findings, and can open a draft PR for safe, low-risk fixes.

F#-specific adaptations:
- Phase 1 discovery rewritten for the fsharp layout: case-insensitive search
  (-iname) catches the capital-.Targets crown jewels (Microsoft.FSharp.Targets,
  Microsoft.Portable.FSharp.Targets); prioritizes the shipped F# SDK build
  logic in src/FSharp.Build, src/fsc, src/fsi and the vsintegration/shims;
  excludes .dotnet/ (the downloaded SDK), obj/bin/artifacts/packages.
- Removed the nonexistent shared/reporting.md import; report template is inline.
- Down-weighted the NuGet build/ rules (D-2 and all of E) that don't apply here
  (fsharp ships via the .NET SDK / FSharp.Build, not NuGet build/ folders) and
  emphasized Categories A, B, C, D-1/3/4/5.
- House-style frontmatter: default copilot engine, network [defaults, dotnet],
  read-only permissions with safe-outputs for writes. Labels automation +
  Area-ProjectsAndBuild (both already exist in dotnet/fsharp).
- Phase 6 build validation scoped (no full ./build.sh) to fit the 30-min timeout.

Lock file compiled with gh aw v0.76.1, matching the repo's existing workflows.

Author: 0101