Add sleap-nn entry points

[gitttt-1234]

This PR introduces a unified CLI interface for SLEAP-NN through a new sleap-nn command that provides subcommands for training (train), tracking (track), and evaluation (eval) workflows. The implementation maintains full backward compatibility with existing entrypoints (sleap-nn-train, sleap-nn-track, sleap-nn-eval).

# Install the package with extras
uvx --from sleap-nn sleap-nn[torch-cpu]

# Then run your command
uvx sleap-nn train --config-name config.yaml --config-dir configs/

[codecov]

Codecov Report

❌ Patch coverage is 97.08738% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.37%. Comparing base (ff91433) to head (9674f8b).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_nn/cli.py	96.73%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
+ Coverage   95.28%   95.37%   +0.08%     
==========================================
  Files          49       49              
  Lines        6765     6978     +213     
==========================================
+ Hits         6446     6655     +209     
- Misses        319      323       +4     

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