feat(setup): first-run welcome guide for the contributor-growth family (#1658)

[Kaap10]

Summary

• Adds a first-run welcome guide for the contributor-growth skill family (plugins/magpie-setup/skills/setup/setup_preflight/guides/contributor-growth.md), following the template established in feat(setup): first-run welcome guides, general and for the security skills #1653 and Welcome guide: utilities family #1663.
• Introduces the typical contributor-growth flow (activity-sweep → identity-map → contributor-to-committer → nomination → candidate-screen → committer-onboarding; sentiment), highlighting private governance (<private-list> confidentiality), calibrated thresholds, and the core principle: information only, never a readiness verdict.
• Provides plain-language invocation prompts across all 9 skills in the family, including calibrate and onboarding-concierge.
• Adds preflight-floor eval case-11-contributor-growth-first-run, re-renders case-9-first-magpie-run to include contributor-growth in the general guide, links the guide in docs/contributor-growth/README.md, and expands test coverage in test_welcome.py.

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other: Framework setup preflight welcome guide

Test plan

• prek run static checks pass (markdownlint, mixed-line-ending, doctoc, add-license-headers)
• For Python packages touched: pytest and ruff check pass cleanly
• For Groovy bridges touched: command-line invocation tested end-to-end
• For skill changes: eval suite passes for the affected skill
  (PYTHONPATH=tools/skill-evals/src python3 -m skill_evals.runner tools/skill-evals/evals/<skill>/)
• For skill behaviour changes: a new or updated eval fixture is included in this PR
  (a regression test for the bug fixed / the behaviour added — see CONTRIBUTING.md)
• Other: python tools/dev/render-preflight-fixtures.py --check passes with zero drift; pytest tools/setup-preflight/tests/test_welcome.py passes (50 tests).

RFC-AI-0004 compliance

• HITL — any new mutation is gated on explicit user confirmation
• Sandbox — no new unrestricted host access; network reach declared in the adapter
• Vendor neutrality — placeholders (<PROJECT>, <tracker>, <upstream>, <security-list>, <private-list>) used in all skill / tool prose (the check-placeholders prek hook is the mechanical gate)
• Conversational + correctable — agentic-override path documented if behaviour is adopter-tunable
• Write-access discipline — no autonomous outbound messages; drafts only, sent on confirmation
• Privacy LLM — private content does not reach a non-approved LLM; redactor invoked where needed

Linked issues

Closes #1658 (child of #1654)

Notes for reviewers (optional)

• The guide is 54 lines of Markdown, well under the 70-line limit enforced by test_each_guide_is_short_and_ends_with_the_way_back.
• case-11 uses the exact surface_hash (sha256:bdf92c88a304ddea) declared by contributor-to-committer.
• Both case-11 and case-9 report fixtures were dynamically generated via render-preflight-fixtures.py ensuring zero drift.

[Kaap10]

Ready for review!

[potiuk]

Adds the contributor-growth welcome guide covering all nine skills, with the standard README block; the skills and the flow it describes check out.

I pushed two commits rather than send this round again:

• bd56c7e — fixup. "Configure it" described private-list access and thresholds without naming a file to configure; it now names the file each skill reads (committer-readiness.md, contributor-nomination-config.md, committer-onboarding-config.md, onboarding-concierge-config.md, contributor-sentiment-config.md, and privacy-llm.md for calibration's private-list reads), and says /magpie-setup:setup config configures rather than "personal install". I also added the next-run test every other family guide has: the nine skills resolve to contributor-growth, and the guide follows the general one on the next run.
• b48fa50 — merge of main. The mentoring, pairing, issue and PR-management guides landed first and claimed the same slots, so this fixture is now case-15-contributor-growth-first-run, the setup case counts go to 135, the test lists carry every family, and the case-9 report is re-rendered.

The PR merges automatically once CI is green. Thanks for the guide!

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