fix: allow all-evals sweep artifact reuse

[Oseltamivir]

Summary

• allow full-sweep PRs with all-evals to use /reuse-sweep-run
• keep evals-only reuse-incompatible
• update workflow gating, merge helper, tests, and policy documentation

Testing

• uv run --python 3.11 --with pytest --with pyyaml --with pydantic python -m pytest utils/test_find_reusable_sweep_run.py utils/changelog_gate_tests/test_run_sweep_gating.py utils/changelog_gate_tests/test_validate_perf_changelog.py -q
• uv run --python 3.11 --with pytest --with pyyaml --with pydantic python -m pytest utils/changelog_gate_tests/ -q
• actionlint .github/workflows/run-sweep.yml
• bash -n utils/merge_with_reuse.sh
• live selector check against PR [NV] Add MiniMax M3 B300 Dynamo vLLM recipes #1863 and run 27951287552

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Note

Medium Risk
Changes CI merge-time sweep reuse gating and the supported merge helper path; wrong policy could skip needed benchmarks or accept stale artifacts, though evals-only remains blocked and tests encode the new rules.

Overview
Full-sweep PRs with the all-evals modifier can now use /reuse-sweep-run and merge_with_reuse.sh, instead of being treated like evals-only runs that must re-benchmark.

The reuse-sweep-gate job in run-sweep.yml no longer blocks on the all-evals label—only evals-only (including PRs with both modifiers) still skips the reuse gate and cannot reuse artifacts. find_reusable_sweep_run.py defaults --reuse-incompatible-label to evals-only only, and the merge helper drops its early exit for all-evals.

AGENTS.md and .github/workflows/README.md document the split: all-evals full sweeps are reusable; evals-only runs are not. Gating and reuse tests are updated to match (e.g. full-sweep-enabled + all-evals + authorized reuse now skips setup).

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