docs: add CONTRIBUTING.md

[aaronjmars]

What

Adds a CONTRIBUTING.md and links it from the README header (a PRs welcome badge). The guide collects the conventions already used across the repo into one place, organized by the four contribution types: a new skill, a new LLM gateway, a community skill pack listing, and core fixes.

It documents:

• Skills — the frontmatter contract (requires:/mcp: two-tier semantics), the scaffolding commands (./new-from-template, ./add-skill, create-skill, ai-build issues), the sandbox-fallback requirement, and the ./generate-skills-json regen step the CI gate enforces.
• Gateways — the existing five-file checklist (native vs sidecar tiers), linking to the README's #adding-a-gateway section.
• Skill packs — the README-row + skill-packs.json + skills-pack.json manifest requirements.
• CI gates — what ci-skills-json and ci-capabilities-parity enforce and how to satisfy them locally.
• PR conventions: branch from main, one change per PR, squash-merge expectations.

Why

A 510-star repo with 170 forks and active external contributors (e.g. the open BEAMR gateway PR #418) had no CONTRIBUTING.md. Every contributor was reverse-engineering branch naming, the CI gates, the skill-submission flow, and the gateway pattern from existing PRs. Surfaced as the top community-health gap in yesterday's repo-actions audit. Lowering fork/contribution friction is a direct lever on ecosystem growth.

Changes

• CONTRIBUTING.md (new): full contributor guide, anchored to real files and README sections — every internal link verified to resolve.
• README.md: added a PRs welcome shield linking to CONTRIBUTING.md in the header badge block.

Notes

• Docs-only — touches no skills/**, aeon.yml, or taxonomy paths, so neither CI gate is triggered; no skills.json regen needed.

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