[codex] Document WarmTestRunner workflow

[terasakisatoshi]

Summary

• Document the Tensor4all.jl WarmTestRunner workflow in AGENTS.md.
• Clarify that Tensor4all.jl tests should use --project=. so the repo environment loads the developed ~/.julia/dev/WarmTestRunner dependency.
• Split the guidance into standard warm-loop usage and 8-worker split_testsets=true usage.
• Record local timing guidance and explain the difference between WarmTestRunner RunSummary totals and external wall time.
• Add daemon/source inspection and stop() guidance for stale daemons, worker-count changes, source changes, and binary rebuilds.

Validation

• Ran git diff --check -- AGENTS.md.
• Documentation-only change; package tests were not rerun for this PR.

Notes

The local worktree still contains an unrelated untracked ext/WarmTestRunner.jl/ checkout, which is intentionally not included in this PR.