feat(decision): add opt-in --vcs-bundle option to decision task

[hneiva]

Add a --vcs-bundle flag to taskgraph decision that writes a native git/hg bundle of the checkout (full history, all refs) to public/vcs.bundle, letting downstream tasks seed a checkout from it instead of re-cloning. The flag is off by default. Bundle creation lives on the Repository abstraction via a new create_bundle method.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.57%. Comparing base (a8a3f4d) to head (b05c197).

Files with missing lines	Patch %	Lines
src/taskgraph/util/vcs.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #987      +/-   ##
==========================================
+ Coverage   78.42%   78.57%   +0.15%     
==========================================
  Files         132      132              
  Lines       12370    12407      +37     
  Branches     1491     1494       +3     
==========================================
+ Hits         9701     9749      +48     
+ Misses       2281     2266      -15     
- Partials      388      392       +4     

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