feat(dx): add repository health check and environment doctor script (#721)

[ravendevhub]

Summary

Resolves #721 by introducing a comprehensive, non-destructive environment doctor script (scripts/doctor.sh) that audits local runtime prerequisites, package managers, blockchain CLI tools, and configuration templates before contributors start development.

Changes

1. Prerequisite & Toolchain Audits:
   • Checks required runtimes: Node.js (>=18), npm, Rust toolchain, and Cargo.
   • Checks container and blockchain CLI tooling: Docker, Docker Compose, Stellar CLI / Soroban, and pnpm.
2. Actionable Feedback:
   • Missing dependencies print clear, actionable installation guidance with direct URLs and commands.
   • Script never mutates developer state or executes sudo commands.
3. Repository Configuration:
   • Verifies presence of .env and critical component package.json / Cargo.toml manifests.
4. Documentation:
   • Linked ./scripts/doctor.sh under CONTRIBUTING.md Prerequisites.

Acceptance Criteria

• Required runtimes are checked.
• Required package managers/tools are checked.
• Missing prerequisites produce actionable messages.
• The script does not modify the developer's environment automatically.