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

[pszymkowiak]

Thanks for the contribution @gula00! Great work on the tail -N rewrite support.

I pushed two small fixes to your branch before merging:

1. Trailing newline preservation — apply_line_window() was using str::lines().join("\n") which drops the final \n. Now preserves it when the input has one.
2. Missing test — Added coverage for tail --lines 7 file (space form without =), the TAIL_LINES_SPACE regex was untested.

All 718 tests pass. Merging into develop.