Optimize the mentoring skill family

[potiuk]

Part of #1342 — apply the setup-family optimization recipe to the mentoring family.

4 skills, 14,455 body tokens in total; 4 over the 200-token always-on budget, 0 over the 5,000-token body budget.

skill	body tokens (budget 5,000)	always-on tokens, est. (budget 200)	lines (cap 500)
good-first-issue-sweep	4,124	~225 ⚠	372
good-first-issue-author	3,611	~273 ⚠	277
newcomer-issue-explainer	3,495	~234 ⚠	302
mentoring-welcome	3,225	~203 ⚠	249

Body tokens are from docs/mode-economics.md; always-on figures are a chars ÷ 4 estimate of description + when_to_use. ⚠ marks a budget exceeded. Line counts include the generated pre-flight block.

Order of work (per the umbrella):

1. Trim the frontmatter of every skill marked ⚠ in the always-on column.
2. Split any over-cap body, moving sub-action-specific rules into sibling files byte-for-byte.
3. Wording pass on the rest, with headings, golden-rule headlines, code blocks and the pre-flight block kept byte-identical.
4. Run each touched skill's eval suite before and after, outside the sandbox.

Record any inconsistencies found but not fixed (eval-coupled wording, stale headings) in the PR description.

[anbv29]

Hi! I’m interested in the mentoring-skill optimization. I’ll begin by mapping the current behavior and suggesting a small measurable improvement. Would that be a useful approach?

[potiuk]

Again - use the optimize skill for that - it should guide you step-by-step and you can start one skill at a time.