Update .gitignore for Phase 0 build artifacts

Add patterns to ignore:
- All build_* directories (build_test, build_profile, build_tsan, etc.)
- Profiling data generated by Phase 0 scripts
- Benchmark result CSV files
- perf and cachegrind output files

This keeps the repository clean while allowing Phase 0 infrastructure
templates to be tracked.

Author: claude

.gitignore

@@ -2,6 +2,7 @@ cmake-build-*/
 docker/
 ldata/
 build/
+build_*/
 dist/
 _build/
 _generate/
@@ -40,4 +41,15 @@ coverage.xml
 # Temporary files
 *.tmp
 *.bak
-*~
+*~
+
+# Phase 0 profiling artifacts (keep templates, ignore generated data)
+docs/baseline/reports/*.txt
+docs/baseline/reports/*.out
+docs/baseline/reports/*.data
+docs/baseline/flamegraphs/*.svg
+*_benchmark_results.csv
+*.prof
+perf.data
+perf.data.old
+cachegrind.out*