docs: refresh debug and config resolution help

[haosenwang1018]

Summary

• document merged default config loading in the README and CLI help
• add the new --debug flag to help output and debugging examples
• align the user-facing docs with the current runtime behavior

Problem

Several recent UX improvements are implemented but not yet reflected in the docs/help output:

• default config discovery now merges multiple config files
• the CLI now supports a one-off --debug flag

Without doc updates, users still see stale config resolution behavior and may miss the new debug path.

Changes

• update the README config resolution section to describe single-file explicit configs vs merged default configs
• add --debug to the CLI help output and environment variable list
• add a one-off debug example in the README connection pooling section

Validation

• git diff --check
• npx tsc --noEmit
• npx @biomejs/biome check src/index.ts README.md