fix: Handle panic in ReadKey()

[liamg]

Fix a panic in Vim character-search commands (f, F, t, T) when ReadKey() receives an empty read or EOF while waiting for the target character.

In that case, ReadKey() could attempt to index into an empty rune slice and panic. This surfaced downstream during interactive use.

What changed

internal/core/Keys.ReadKey() now:

• handles empty reads without indexing into an empty buffer
• treats EOF as an abort condition instead of panicking
• continues waiting when a read returns no bytes but input has not ended

Tests

Added regression tests covering:

• empty read followed by valid input
• EOF returning an abort result without panicking

Why

Vim-style motions should fail gracefully if the follow-up character is unavailable; they should never crash the shell.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.84%. Comparing base (0637db4) to head (cd0f960).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/core/keys.go	50.00%	11 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   33.03%   32.84%   -0.19%     
==========================================
  Files          57       58       +1     
  Lines        9109     9225     +116     
==========================================
+ Hits         3009     3030      +21     
- Misses       6062     6155      +93     
- Partials       38       40       +2     

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[maxlandon]

Thanks a lot your PR is awesome !!!

I definitely have to merge all your PRs, I will do this before the end of the month!