🎯 Minimal-code ladder in /feature Phase 1

[PierreBrisorgueil]

Add a short decision ladder to the /feature skill's Phase 1, applied before writing any NEW code, to bias toward minimal, necessary implementation:

1. Must it exist? (YAGNI)
2. Already in the codebase?
3. Available in the standard library?
4. Native platform feature?
5. Already an installed dependency?
6. One-line solution?
7. Only then: minimum viable code.

Validation, security and accessibility stay non-negotiable — the ladder prunes gratuitous code, never guardrails.

Sub-issues track the same ~8-line insertion in each stack repo's /feature skill (identical, generic wording).

Created via /dev:issue