Optimize the contributor-growth skill family

[potiuk]

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

6 skills, 28,197 body tokens in total; 3 over the 200-token always-on budget, 1 over the 5,000-token body budget.

skill	body tokens (budget 5,000)	always-on tokens, est. (budget 200)	lines (cap 500)
contributor-sentiment	4,726	~233 ⚠	463
onboarding-concierge	3,374	~267 ⚠	318
contributor-activity-sweep	3,323	~203 ⚠	366
committer-onboarding	7,310 ⚠	~164	712 ⚠
contributor-nomination	4,761	~179	461
contributor-to-committer	4,703	~185	505 ⚠

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.