feat(cli): support STELLAR_EXPLAIN_TOKEN

[pengyuayn]

Summary

• Adds a small env helper for STELLAR_EXPLAIN_TOKEN.
• Sends the token as an Authorization: Bearer ... header for CLI API requests.
• Allows an explicit apiToken option to override the environment value in tests/consumers.
• Removes stale stub code in src/index.ts so the CLI entrypoint typechecks/builds.

Validation

• PASS: npm test -- --run tests/client.test.ts
• PASS: npm run typecheck
• PASS: npm run build

Closes #563