presubmit: Fast-path nested PRESUBMITs in agents/, gpu/, and content/test/gpu/ (part 6 of 8)

Use InputApi.HasAffectedFiles() to early-return from test and linter
runners when no relevant files are affected:
- agents/ (common, extensions, infra/review_rag_indexer, prompts,
  skills, testing)
- content/test/gpu/
- gpu/ (PRESUBMIT.py, command_buffer, ipc/common, vulkan)

Benchmark (content/test/gpu/DIR_METADATA, content/test/gpu/ PRESUBMIT):
- Baseline (origin/main): 24.69s ± 0.49s
- Optimized (CL branch):  241ms ± 3ms
- Savings: -24.44s (-99.0%, 102.29x speedup)

Overall series benchmark (all 8 CLs combined):
- Multi-subsystem change (touching files across all 8 subsystems):
  Baseline (origin/main): 60.44s ± 0.74s
  Optimized (series):     14.17s ± 0.20s
  Savings: -46.26s (-76.5%, 4.26x speedup)
- Standard root build edit (build/config/compiler/BUILD.gn):
  Baseline (origin/main): 569ms ± 8ms
  Optimized (series):     470ms ± 6ms
  Savings: -100ms (-17.5%, 1.21x speedup)

Bug: 534712288
Test: python3 -m py_compile agents/**/PRESUBMIT.py content/test/gpu/PRESUBMIT.py gpu/**/PRESUBMIT.py
Change-Id: Ic1d2ed39eabd9df27787554e5e9691b8bca92a2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8414400
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1703169}
NOKEYCHECK=True
GitOrigin-RevId: 0fd453d44c7e8ac6a691e92b4a73170d49132ea6