Add CLAUDE.md with repository guidance for AI assistants

[yir-arch]

Summary

Adds a CLAUDE.md to give future AI assistants accurate context about this repository.

The key insight captured: this is the public distribution and issue-tracking repo for GitHub Copilot CLI — it contains no application source code, no build system, and no test suite. The actual CLI ships as a precompiled binary via GitHub Releases and package managers. The doc steers assistants away from hunting for source/package.json/Makefile that don't exist.

What the CLAUDE.md covers

• What lives here: install.sh, README.md, changelog.md, LICENSE.md, issue templates, and issue-triage workflows.
• Changelog conventions: newest-first ## <version> - <date> blocks, non-contiguous versions, flat present-tense bullet lists, backticked inline code.
• Install script contracts: curl/wget + root/non-root support, PLATFORM/ARCH detection, SHA256SUMS.txt validation, and the VERSION/PREFIX/GITHUB_TOKEN env vars.
• Issue automation: clarifies these workflows manage the issue tracker (not product CI/CD); winget.yml is the only release-adjacent one.
• Product questions: points to README.md + changelog.md for CLI behavior (models, LSP config, MCP, auth) since there's no source to read.

https://claude.ai/code/session_01CwM5HJhmNDcBNyniUDetbC

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