feat: add db-schema subcommand for exporting database schema SQL

[cxhello]

Summary

• Add nacos-setup db-schema subcommand to export full database schema SQL for a given Nacos version
• Support MySQL and PostgreSQL with correct filename mapping (pg-schema.sql for PostgreSQL)
• Local-first resolution: installed Nacos → cache → GitHub remote download with new/old path fallback
• Interactive mode when args are missing, non-interactive safe ([ -t 0 ] detection)
• All log output to stderr, schema SQL to stdout (pipe-friendly)

Closes #25 (Part B)

Usage

# Full parameter mode
nacos-setup db-schema -v 3.2.0 --type mysql

# Pipe directly to database
nacos-setup db-schema -v 3.2.0 --type mysql | mysql -h localhost -u root -p nacos

# Interactive mode
nacos-setup db-schema

Test plan

• 19 unit/integration tests added (tests/test_db_schema.sh)
• Full test suite passes (113/113)
• Smoke tested: MySQL 3.2.0 (new path), MySQL 3.1.1 (old path fallback), PostgreSQL 3.2.0
• Verified stderr/stdout separation
• Test on Linux environment

[qiacheng7]

cool，is it compatible with Windows?

Verify that full init SQL and incremental schema-change SQL can be output to a local temp folder, with the matching commands printed in the terminal; DBs: cover PostgreSQL and MySQL

[cxhello]

Thanks for the review!

Windows compatibility: Added in the latest commit (cc8eb76). windows/lib/db_schema.ps1 mirrors the bash implementation, tested with pwsh 7.6.0 on macOS.

Incremental schema-change SQL: Not included in this PR. Nacos upstream doesn't provide versioned migration SQL files — only full schema per release. Generating reliable ALTER statements from diffing two full schemas is error-prone (can't handle renames, data migrations, etc.). Happy to revisit if Nacos adds official migration scripts in the future.

Output to temp folder with commands: Currently outputs to stdout for pipe-friendly usage (db-schema ... | mysql ...). We could add a --output flag to save to a file and print the matching command — would you like that as part of this PR or as a follow-up?