fix(cli): fix cargo subcommand parsing and update README

[rocketman-code]

Summary

• Simplify cargo subcommand detection: always strip argv[1] when it's "avail", matching the standard pattern used by cargo-edit, cargo-nextest, etc. The previous approach inspected argv[0] basename, which was unnecessarily complex.
• Remove allow_hyphen_values from clap — hyphen-starting names can be passed after -- instead, which is the standard CLI convention.
• Add tests for subcommand stripping, flags after positional args, and -- separator.
• Update README: add crates.io badge, fix stdin example portability, use short flags, remove unreachable match arm from library example.

Test plan

• cargo test passes (all existing + new CLI tests)
• Manual: cargo avail std shows "reserved"
• Manual: cargo avail --json std outputs valid NDJSON
• Manual: cargo avail -- ---test reports invalid name