Validate via the hub's composite action; standalone tools; delta-over-AGENTS.md TODOs

[kahwaipd]

• `.github/workflows/validate.yml` is now a single step: `uses: Virtual-Protocol/butler-skills/.github/actions/validate@main` (checks out the skill, fetches the hub tooling into `$RUNNER_TEMP`, installs viem, runs `validate.py --standalone .` and, when `duty.py` exists, `replay.py`). The action lands with the butler-skills PR; this repo's CI is red until that merges.
• README: validate locally with `curl -sSLO https://virtual-protocol.github.io/butler-skills/tools/{validate.py,replay.py}\` — nobody clones the registry; `butler-` is the maintainer prefix, `bevo-` names are refused.
• `.gitignore` keeps the downloaded tooling (validate.py, replay.py, stub_bevo.py, fixtures/) out of the skill.
• SKILL.md TODO comments state the rule: a skill is the delta over AGENTS.md — never restate grammar, invariants, budgets or routing.

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