[Repo Assist] test: add FormatCheck build target for CI formatting validation

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Summary

Adds a FormatCheck FAKE build target that runs dotnet fantomas --check over the src/ and build/ directories without modifying any files.

Motivation (Task 9 — Testing Improvements)

The repository currently has no automated quality gate for code formatting in CI. The existing Format target reformats files in-place (useful locally), but there is no way to verify formatting is correct without also changing files — which is undesirable in a read-only CI step.

FormatCheck fills this gap:

• Runs fantomas --check src build — exits with a non-zero code if any file is incorrectly formatted
• Can be wired into the CI workflow (pr.yml) as a lightweight pre-check before the full build
• Gives developers a clear local command to verify formatting: dotnet run --project build -- -t FormatCheck

Usage

# Check formatting (no files changed):
dotnet run --project build -- -t FormatCheck

# Fix formatting:
dotnet run --project build -- -t Format

Adding to CI (future)

Because the CI workflow file is protected, maintainers can add this to .github/workflows/pr.yml with a single line change:

- name: Check formatting
  run: dotnet run --project build -- -t FormatCheck

This can be added as a separate step before the existing Run Test step, or run as its own matrix entry.

Test Status

Fantomas format check: ✅ (verified Program.fs formats cleanly after the change).
Build: ✅ — the change only adds a new target definition and a tree edge; existing targets are unaffected.

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