docs: add Ponytail guidance

[BenSheridanEdwards]

Why

Ponytail should be visible in the shared agentic infrastructure template as the default simplicity discipline: delete first, reuse existing patterns, and avoid adding abstractions or dependencies unless they earn their keep.

What Changed

• Added Ponytail simplicity guidance to agent instructions.
• Listed Ponytail alongside the external agentic tools in the README and tech stack.
• Taught the setup script and installer skill to check/document Ponytail without vendoring a stale skill copy.

Proof

• pnpm install
• pnpm setup:agents
• pnpm verify
• pnpm e2e
• Visual proof attached or linked, or N/A with reason: N/A, docs/setup-script guidance only; no rendered UI changed.
• StyleProof reviewed, or N/A with reason: N/A, no rendered surface changed.
• Other proof: pre-commit and pre-push hooks passed without bypassing.

CI Status

• Latest-sha GitHub Actions are green.
• Pending checks are named below: none.
• Failing checks are named below with the fix plan: none.

Risk And Rollback

Low risk. This changes guidance and setup validation only; rollback is reverting the docs/script commit.

Definition Of Done

• Behaviour is covered by the E2E map when user-visible behaviour changed.
• Logic is unit-tested where logic changed.
• Accessibility was considered and verified for interactive/UI changes.
• The diff follows .agents/project/ARCHITECTURE.md and CONVENTIONS.md.
• Any new architectural decision is recorded as an ADR.
• No gates were bypassed or weakened.

[github-actions]

Fallow audit report

Found 1 finding.

Details

Severity	Rule	Location	Description
minor	fallow/unused-dev-dependency	package.json:44	Package '@lhci/cli' is in devDependencies but never imported

Generated by fallow.

[github-actions]

Fallow audit report

0 inline findings selected for GitHub review.

[github-actions]

🗺️ StyleProof report

✓ No visual changes detected.