Improve whitespace guidance and tooling

[johnml1135]

Summary

• document the required local CI parity tasks for whitespace and commit-message checks
• clarify that whitespace validation is commit-history based
• preserve UTF-8 BOM behavior in the local whitespace fixer so it does not introduce spurious diffs

Validation

• CI: Full local check

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This change is 

[Copilot]

Pull request overview

Documents and aligns local developer/agent guidance with CI whitespace and commit-message checks, and updates the local whitespace fixer to preserve UTF-8 BOM to avoid unnecessary diffs.

Changes:

• Preserve UTF-8 BOM when rewriting files in fix-whitespace.ps1.
• Clarify that whitespace validation is based on commit history (origin/main..HEAD) in check-whitespace.ps1.
• Update repository/agent instructions to prefer VS Code “CI:*” tasks for local CI parity.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Build/Agent/fix-whitespace.ps1	Detect and preserve UTF-8 BOM when re-writing files; improve user guidance output.
Build/Agent/check-whitespace.ps1	Clarify commit-range semantics and remediation steps in whitespace failure output.
AGENTS.md	Add CI parity task guidance and update the validation checklist.
.github/instructions/terminal.instructions.md	Prefer CI parity VS Code tasks over ad-hoc shell commands.
.github/instructions/repo.instructions.md	Add succinct CI parity rules for whitespace and commit-message checks.

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   5m 50s ⏱️ +6s
4 058 tests ±0  3 987 ✅ ±0  71 💤 ±0  0 ❌ ±0 
4 067 runs  ±0  3 996 ✅ ±0  71 💤 ±0  0 ❌ ±0 

Results for commit fc536e0. ± Comparison against base commit d58727c.