fix: handle tail rewrites with read tail-lines

[gula00]

Closes #407

Summary

This PR fixes the tail -N rewrite regression where:

• rtk rewrite "tail -20 <file>" incorrectly produced rtk read -20 <file>
• the rewritten command then failed because rtk read does not accept that flag format

What changed

• Added tail-specific rewrite handling in src/discover/registry.rs:

  • tail -N <file> -> rtk read <file> --tail-lines N
  • tail -n N <file> -> rtk read <file> --tail-lines N
  • tail --lines=N <file> -> rtk read <file> --tail-lines N
  • tail --lines N <file> -> rtk read <file> --tail-lines N

• Added --tail-lines support to rtk read in:

  • src/main.rs
  • src/read.rs

• Made --tail-lines and --max-lines mutually exclusive

• Kept unsupported tail forms (e.g. tail -c) as no-rewrite fallback for safety

Validation

• Added unit tests for rewrite mappings and read tail behavior

• Ran:

  • cargo fmt --all
  • cargo clippy --all-targets
  • cargo test --all