ci(bench): remove the walltime job

[lcottercertinia]

📝 Overview

The walltime job runs on a CodSpeed macro runner, and the org does not allow those runners on public repositories. A dispatch of the Benchmarks workflow queues the job and it never starts. The queued job, and CodSpeed's CodSpeed Macro Runners check (action_required), then attach to the dispatched commit and leave its pull request unmergeable, as run #85 did to #128. Enabling it would mean an org-level runner setting for one repository, so the job goes.

🛠️ Changes

• benchmark.yml: remove the walltime job. workflow_dispatch stays, so the simulation run can still be started by hand.
• .github/workflows/README.md: drop walltime from the trigger table and the description; one "No walltime job" row replaces the two walltime rows, and points to pnpm run bench:large for wall time.
• AGENTS.md, benchmarks/AGENTS.md: drop the sentence about the manual walltime job.

🧩 Type of change

• 🐛 Bug fix — fixes an issue
• ✨ Feature — adds new functionality
• ♻️ Refactor — no behaviour change
• ⚡ Performance
• 📝 Docs
• 🔧 Chore — tooling, CI, deps
• 💥 Breaking change

🔗 Related issues

None filed. Unblocks #128, whose head carried the queued dispatch (since cancelled).

✅ Checklist

• Tests added/updated — CI config and docs only; nothing to test.
• pnpm run ci passes locally — biome and both typechecks pass; 4 tests in src/next/__tests__/Worker.test.ts fail in my environment (Node 22.22, the worker cannot import api/hosts.js from .ts source). They fail identically on main without this change.
• Changeset added (pnpm changeset) if this affects the published package — does not apply; nothing published changes.
• Docs updated if behaviour or the public API changed

🤖 Generated with Claude Code

https://claude.ai/code/session_01QDthfCKoDiYWqy5FwMeJPe

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Generated by Claude Code

[codspeed]

Merging this PR will not alter performance

✅ 3 untouched benchmarks

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_{Comparing ci/remove-walltime (09ad930) with main (0785c68)}