feat: add default ignore patterns for protobuf-generated files

[svarlamov]

Summary

Adds 9 protobuf-generated file patterns to DEFAULT_IGNORE_PATTERNS in src/authorship/ignore.rs so that protoc output is excluded from git-ai tracking by default. Previously, these files were counted as human-authored code, inflating human line counts and diluting AI acceptance rates.

New patterns: *.pbobjc.h, *.pbobjc.m, *.pb.go, *.pb.h, *.pb.cc, *_pb2.py, *_pb2_grpc.py, *.pb.swift, *.pb.dart.

Java/Kotlin protobuf outputs are intentionally excluded—they generate ordinary .java/.kt filenames with no distinguishing suffix. Users can handle those via .gitattributes linguist-generated or .git-ai-ignore, which are already supported.

Closes #1103

Review & Testing Checklist for Human

• Behavior change: this silently changes defaults for existing users who have protobuf files in their repos. Confirm this is the desired default (users can still override via .git-ai-ignore or config).
• False-positive risk on *.pb.h / *.pb.cc: verify there are no common non-protobuf naming conventions that use .pb. as a double extension in C++ headers/sources.
• Spot-check that the glob patterns work correctly for both bare filenames and nested paths (unit tests cover this, but worth a manual sanity check on edge cases).

Notes

• Developed TDD-style: tests were written first and confirmed failing, then the patterns were added.
• The integration test covers all 9 patterns across realistic nested directory structures and is also reused in the worktree variant via reuse_tests_in_worktree!.

Link to Devin session: https://app.devin.ai/sessions/f4b349e42ecd4f5081c5876e28a5093f
Requested by: @svarlamov

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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