chore: add community health files and CI improvements

[dihannahdi]

Summary

Adds essential community health files and CI hardening for the ucp-schema project.

Changes

New Files

• CONTRIBUTING.md — Comprehensive contributing guide covering prerequisites, development workflow (Makefile targets, feature flags, project layout), PR process with conventional commits, coding standards, testing guidelines, and CLA instructions.
• SECURITY.md — Security policy with vulnerability reporting instructions (GitHub Security Advisories), response timeline, and scope definition.
• .github/dependabot.yml — Automated dependency updates for both Cargo crates and GitHub Actions (weekly schedule, conventional commit prefixes).

CI Improvements (ci.yml)

• MSRV check job — Installs Rust 1.70 (the declared
  ust-version) and runs \cargo check --all-targets\ to catch accidental use of newer language features.
• No-default-features job — Builds and tests without the
  emote\ feature flag to verify the codebase compiles correctly in offline-only mode.
• Updated the \�uild\ job's
  eeds\ to gate on the two new jobs.

Category

• Core Protocol
• Infrastructure
• Documentation
• UCP Schema
• Community Health (.github)

Checklist

• I have read the contributing guidelines
• My changes follow the project's coding standards
• I have tested my changes locally
• CI checks are expected to pass