docs(platforms): 📝 add platform setup guides

[alsd4git]

Summary

This PR reorganizes the README and adds focused setup guides for Debian/Ubuntu, macOS, and Windows.

• Document installation, package flows, local overlays, recovery, and verification for each platform.
• Keep the Windows path clearly marked as preview and describe its current limitations.
• Reject unsupported Linux distributions before a real run links or copies managed files.
• Add the la listing command to Bash and Zsh so it matches the documented shell commands.
• Extend the Ubuntu smoke test to cover la and retain the Bash 3.2 compatibility check.

Validation

• shellcheck -x on the installer, libraries, shell files, and smoke tests
• shfmt -d -i 4 -ci -bn on the shell sources
• ./tests/test-installer-functions.sh
• ./tests/docker-ubuntu-smoke.sh
• Alpine guard check confirms unsupported hosts exit before managed-file changes

The intended squash commit title is the PR title above.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed2026-08-29T00:08:37.946980Z	369bccc	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit:369bccc0bd

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