AutoMerge: Add AutoMerge guideline to reject Git LFS

[DilumAluthge-LLM]

Summary

Add a new AutoMerge guideline that rejects package registrations using Git LFS.

This runs for both new packages and new versions, and fails when the extracted package tree contains either:

• a Git LFS pointer file
• a .gitattributes entry configuring LFS

Testing

• Ran focused Julia checks for the new Git LFS detection logic and guideline ordering.
• julia --project=AutoMerge -e 'using Pkg; Pkg.test()' still hits an existing local juliaup environment issue in the pre-existing Version can be added test when resolving the 1.12.5 channel.

CC: @DilumAluthge
🤖 Generated by Codex.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.49%. Comparing base (7baf4fe) to head (2b8d308).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
AutoMerge/src/guidelines.jl	95.00%	2 Missing ⚠️

❌ Your project check has failed because the head coverage (49.49%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (7baf4fe) and HEAD (2b8d308). Click for more details.

HEAD has 13 uploads less than BASE

Flag	BASE (7baf4fe)	HEAD (2b8d308)
	24	11

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #705       +/-   ##
===========================================
- Coverage   78.48%   49.49%   -29.00%     
===========================================
  Files          29       29               
  Lines        1938     1970       +32     
===========================================
- Hits         1521      975      -546     
- Misses        417      995      +578     

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