chore: refactor pipelines to be goroutine-safe

[twelvelabs]

Refactors the Pipeline methods so they can safely be used in coroutines.

• Removes the paths slice from Processor to make it immutable. Needed because we pass pointers to the processors around everywhere.
• Merges Pipeline.Index into Pipeline.Match.
• Updates Pipeline.Match to return a collection of match responses, each containing the matched processor and paths.

[codecov]

Codecov Report

Attention: Patch coverage is 57.69231% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 76.86%. Comparing base (43473c5) to head (ee1b27b).
Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/cmd/files.go	0.00%	5 Missing ⚠️
internal/cmd/init.go	0.00%	4 Missing ⚠️
internal/stylist/pipeline.go	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   76.59%   76.86%   +0.27%     
==========================================
  Files          27       27              
  Lines        2153     1733     -420     
==========================================
- Hits         1649     1332     -317     
+ Misses        463      361     -102     
+ Partials       41       40       -1     

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