Harden installer and add Copilot instructions

[selfish666420]

What does this PR do?

This updates contributor ergonomics and installer reliability so setup is more predictable across environments.

It adds a new .github/copilot-instructions.md with repo-specific build/test/lint/security commands, architecture notes, and key conventions for future coding sessions. It also hardens install.sh by validating --dir usage, supporting wget fallback for checksum fetches, matching checksum entries by exact filename, and handling Windows .exe extraction/target naming correctly. In addition, it fixes a mismatched comment in test-infrastructure/Dockerfile so the documented dependencies align with what the image actually installs.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[DeusData]

Thanks for the installer fixes. The checksum lookup, Windows executable handling, and missing directory-argument validation are useful, but this PR mixes installer behavior, a Copilot instruction file, and a test-container documentation change. Please split the installer hardening into a focused bug PR with regression coverage and move the instruction/documentation work into a separate contribution. Installer and agent-instruction changes receive separate security review.

[selfish666420]

Thanks for the installer fixes. The checksum lookup, Windows executable handling, and missing directory-argument validation are useful, but this PR mixes installer behavior, a Copilot instruction file, and a test-container documentation change. Please split the installer hardening into a focused bug PR with regression coverage and move the instruction/documentation work into a separate contribution. Installer and agent-instruction changes receive separate security review.

Done. I split this PR to installer hardening only and removed the Copilot-instructions and test-container comment changes from this branch. The remaining diff is focused on install.sh reliability and cross-platform handling.