docs(writing-skills): step numbering rule for skills

[WilcoLouwerse]

What

Adds a Step Numbering section to docs/claude/writing-skills.md (under SKILL.md Format) and one matching line to docs/claude/skill-checklist.md.

The rule:

1. Steps are whole numbers from 1 upward, ascending by one, no gaps. There is no Step 0.
2. Document order is execution order. Never "runs after Step 4, despite the number" — renumber instead.
3. At most one trailing letter (Step 7a), only where a step is genuinely a sub-step of the one before it. Letters run a, b, c without gaps.
4. Nothing after the letter, nothing else appended. No Step 5c.5, no Step 6.5, no Step 4a.2.
5. Same rules for the internal steps of a references/*.md sub-procedure and for ordered lists inside a step.

Plus: inserting a step means renumbering — map every old number to its new one first, then apply the whole map in one substitution across SKILL.md, references/, scripts/, evals/ and the learnings files, so no pointer goes stale and no number gets re-mapped twice.

Why

Step numbers are a skill's addressing scheme: every see Step N, every learnings-index pointer, every eval note and every reference-doc title resolves through them. Once the sequence stops being sortable, a reader can no longer tell what has already run at any point.

Observed

hydra's review-pr had drifted to Step 0, Step 0.5 after Step 1, Step 1a/2a after Step 2, Step 5c.5, Step 6.5b and a Step 8b with no 8a. Renumbered to Step 1–27 in ConductionNL/hydra (companion PR: https://github.com/ConductionNL/hydra/pull/681).

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