fix(git-ops): truncate PR body to stay within GitHub's character limit

[EspenAlbert]

Summary

GitHub rejects PR bodies exceeding 65,536 characters. This causes create_or_update_pr and update_pr_body to fail when path-sync generates large command output (e.g., verbose dependency update logs).

Adds a _truncate_body helper that caps the body at 64,536 characters (with a 1,000-char buffer) and appends a truncation notice. Applied in both update_pr_body and create_or_update_pr.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.38%. Comparing base (8edafab) to head (9ea4aa8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
path_sync/_internal/git_ops.py	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   65.55%   65.38%   -0.17%     
==========================================
  Files          24       24              
  Lines        1344     1352       +8     
==========================================
+ Hits          881      884       +3     
- Misses        463      468       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.