Remove stale run artifacts and document commit hygiene

[irvineoy]

Remove 266 files left by individual validation runs: eight job/config helpers, 242 vLLM build reports, and 16 historical FlyDSL reports and summaries. The removed files were archived outside the repository and verified byte-for-byte before deletion.

Update AGENTS.md, CONTRIBUTING.md, the README, and the task authoring/validator guides with explicit staging checks, artifact retention rules, and guidance for keeping one-off configs untracked. Add task-report ignore patterns and replace three task README references to the removed inference config with the reusable validator workflow.

All 459 task config files, kernel sources, workloads, references, and evaluation code are preserved. Maintained validation configs, regression fixtures, and linked qualification records remain in place.

Validation on macOS with CPython 3.12 and GNU coreutils cp on PATH:

OMP_NUM_THREADS=1 VECLIB_MAXIMUM_THREADS=1 python -m pytest -q -p no:cacheprovider \
  tests/test_task_materialization_v2.py \
  tests/test_perf_helper_materialization.py \
  tests/test_triton_task_migration_v2.py \
  tests/test_flydsl_task_migration_v2.py \
  tests/test_quality_loop_runner_contracts.py
# 6927 passed
make check-perf-helpers
# passed

Also verified the exact deletion set, remaining references, new documentation links, report ignore rules, and retained fixture/input visibility. git diff --check passed.

No GPU validation was run; this change removes generated artifacts and updates documentation/ignore rules without changing task execution.