[Good First Issue] Write CONTRIBUTING.md for first-time contributors

[Eras256]

Context
This repo has no CONTRIBUTING.md. Everything a contributor needs (prerequisites, build order, dev/test commands) already exists in docs/CLAUDE.md §1-2 and §7 — but that file is written as an engineering-rules reference for AI-assisted sessions, not an onboarding doc for a first-time human contributor arriving from GitHub/GrantFox.

Scope
Write CONTRIBUTING.md covering:

• Prerequisites (Node ≥ 20, pnpm ≥ 10, Rust + wasm32-unknown-unknown target; Supabase/Stellar CLI optional) and the exact build order (pnpm install, then @contextio/config → @contextio/shared → contextio-sdk before the rest).
• How to run the stack locally in mock mode — works fully offline, no external credentials needed (pnpm dev).
• What "done" means before opening a PR: pnpm lint, pnpm typecheck, pnpm test, pnpm test:e2e, cargo test --all (per docs/CLAUDE.md §2).
• Commit message convention, including the AI co-authorship trailer rule (docs/CLAUDE.md §7).
• Where to find good-first-issue vs. advanced work (GitHub labels).

Acceptance criteria

• A contributor who has never touched this repo can go from git clone to a passing pnpm test using only CONTRIBUTING.md — verify this literally, on a clean checkout, before opening the PR.
• Linked from the root README.md.