docs: document environment loading precedence

[shuaiyafei]

Summary

• Document how local, test, Docker Compose, and GitHub Actions environments load variables.
• Explain precedence between shell/CI variables and dotenv files.
• Document module-load timing so contributors know environment changes require a process restart.

Validation

• npm run typecheck passed.
• git diff --check passed.
• npm run lint is blocked by a pre-existing invalid character in test/routes/ngos.test.ts:241.
• npm run format reports pre-existing formatting issues across the repository and cannot parse the same test file.
• npm test is blocked by the same parse error and unavailable local Postgres (ECONNREFUSED).

Closes #164