Improve action input descriptions and reorder consistently

[smorimoto]

Summary

• Enriched all action input descriptions in action.yml with practical details — accepted values, default behaviour, related inputs, and platform-specific notes — to match the level of detail already present in windows-environment and windows-compiler.
• Reordered inputs consistently across action.yml, README.md, and constants.ts by user relevance: core settings first, then opam behaviour, caching, Windows-specific, and advanced/internal.
• Updated the README inputs table to reflect the improved descriptions and corrected stale default values (e.g. cache-prefix was listed as v1 but is actually v3).

Test plan

• Verify the action runs correctly on all platforms (no behavioural changes, documentation only).
• Confirm the README table renders correctly on GitHub.