Tighten README prose and consolidate option list

[aka-hogelog]

Summary

• Collapsed the three-paragraph intro into two sentences. The "not a native binary compiler" framing is folded into the same line that notes the runtime Ruby dependency.
• Moved options into a single ## Options section with one-line descriptions. Added --path DIR to that list instead of re-enumerating every flag in a trailing sentence.
• Dropped the shebang-passthrough edge-case note.

Kept each usage variant (pack / gem pack / gem install) in its own code block with a short lead-in, rather than one consolidated block.

Test plan

• Render README.md on GitHub and confirm formatting looks right