Add AGENTS.md for Cursor Cloud development environment

[skrashevich]

Adds AGENTS.md with Cursor Cloud specific instructions for future agents working on this codebase.

What's included

• Build and run commands
• Testing commands (go test ./..., go vet ./...)
• Notes on macOS-specific features gracefully degrading on Linux
• LLM/TUI caveats for non-TTY environments

Environment verification

All checks passed on the Cloud Agent VM:

• Build: go build -o godiskanal . compiles successfully
• Tests: All 38 tests pass across 5 packages
• Lint: go vet ./... clean
• Run: Application scans directories and produces expected output

Sample scan output:
godiskanal_scan_output.log

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