feat(skill): code-modernizer with ponytail-derived ladder

[CodeWithJuber]

What & why

Replaces the minimal code-modernization skill with a comprehensive code-modernizer skill that adapts DietrichGebert/ponytail's decision ladder for retroactive code auditing — not just gating new code.

Key additions over the previous skill:

• Scope handshake (Full / Partial / Targeted) — confirms blast radius before generating diffs
• Pre-flight scanner (scripts/preflight_scan.py) — stdlib-only Python that reports file count, LOC, available dependencies, and high-cost files before any work begins
• The ladder — 8-rung decision procedure (YAGNI → stdlib → platform → installed dep → researched new dep → one-liner → minimal code) adapted from ponytail (MIT)
• Mandatory live research for rung 6 (new dependency) — never recommends a package from memory alone
• Reference files: research protocol, design patterns cheatsheet, cost/impact pre-flight rubric
• modernize: markers for deferred opportunities, mirroring ponytail's ponytail: comment convention

The skill composes with ponytail: ponytail governs new code as it's written; code-modernizer audits and modernizes what's already in the repo.

Files changed

File	Change
global/tools/code-modernization/SKILL.md	Rewritten with full ladder, scope control, 5-step procedure
global/tools/code-modernization/references/research-protocol.md	New — library vetting checklist and decision template
global/tools/code-modernization/references/design-patterns-cheatsheet.md	New — smell → pattern → when-to-skip table
global/tools/code-modernization/references/cost-impact-preflight.md	New — pre-flight template and impact/flexibility rubric
global/tools/code-modernization/scripts/preflight_scan.py	New — stdlib-only scanner for file count, LOC, deps, high-cost files

Checklist

• npm test passes (Node 18/20/22) — no runtime code changed
• npm run check passes (Biome lint + format) — markdown/Python only
• Conventional commit message (feat:)
• No new runtime dependency (dev deps ok) — scanner is stdlib-only Python
• Substrate/docs updated if this changes forge substrate, forge impact, router/gate, or MCP substrate tools — skill docs self-contained

Risk & rollback

• Risk level: low — replaces one skill definition with an expanded version; no runtime code, no API changes
• Rollback plan: revert this single commit to restore the previous SKILL.md

Extra checks (tick if applicable)

• If AI-assisted: I understand it, verified the package APIs, and it has tests

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