Add Repository Custom Instructions validation (existence + 4000-char limit)

[Copilot]

New Feature

GitHub Copilot silently truncates or ignores Repository Custom Instructions files that exceed 4,000 characters. This adds an explicit validation script that catches violations across all repositories and halts with a non-zero exit code.

What does this feature do?

• Existence check: fails if no custom instruction files are found in any scanned repository
• Size check: fails if any file exceeds 4,000 chars; emits a timestamped Excel report listing each violation
• All error messages link directly to the GitHub Copilot documentation

Implementation details

validate_custom_instructions.py — follows the same patterns as existing assessment scripts (parallel ThreadPoolExecutor, rate-limit guard, CONFIG dict):

Files checked per repo:
  .github/copilot-instructions.md
  .github/instructions/*.instructions.md

Exit codes:
  0 — all validations passed
  1 — existence or size validation failed

Excel report (custom_instructions_violations_YYYYMMDD_HHMMSS.xlsx) columns: Repository · File Path · Character Count · Limit · Excess Characters · GitHub Copilot Rule. Falls back to CSV when openpyxl is not installed.

test_validate_custom_instructions.py — 19 unittest tests covering: no file found, file within limit, file over limit, exact-boundary (4000 chars valid / 4001 invalid), multi-repo violations, instructions/ directory scanning, Excel output integrity.

Other changes

• .gitignore — added *.xlsx
• README.md / AGENTS.md — documented new script, install step (pip install openpyxl), exit codes, and output format