docs: add Copilot repository instructions

[sterngold]

What changed

Adds cloud-portable GitHub Copilot instructions for the polyglot repository template, including placeholder discipline and exact manifest-aware Make/ShellCheck gates.

Validation

• shared Copilot conformance checker: 8/8 repositories conform
• make build, make lint, make test, and shellcheck scripts/*.sh passed
• no dependency or manifest changes

Copilot review is requested because Claude/Codex authored this PR. CI, unresolved-thread cleanup, and owner approval remain mandatory.

[Copilot]

Pull request overview

Adds a new .github/copilot-instructions.md file to provide cloud-portable GitHub Copilot guidance for this polyglot repo template, emphasizing placeholder discipline, repo-relative execution, and the repo’s manifest-aware Make/CI gates.

Changes:

• Add Copilot agent instructions focused on template-safe behavior and portability.
• Document the intended “no-op” Make targets in the template checkout and the repo’s ShellCheck command.
• Document CI expectations (aggregate ci check, Conventional Commits, gitleaks) for merge readiness.

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 24dba6f5db

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.