fix(gateway): support platform config paths

[RerankerGuo]

Description | 描述

Adds platform-aware gateway config discovery so Linux users can place tdai-gateway.yaml/json under $XDG_CONFIG_HOME/tencentdb-agent-memory (or ~/.config/tencentdb-agent-memory by default), with matching macOS and Windows config-directory fallbacks.

Also makes missing explicit gateway config paths and legacy data-dir fallback warnings emit only once, preventing repeated noisy filesystem-path warnings when optional paths are absent.

Related Issue | 关联 Issue

Fix #103

Change Type | 修改类型

• Bug fix | Bug 修复
• New feature | 新功能
• Documentation update | 文档更新
• Code optimization | 代码优化

Self-test Checklist | 自测清单

• Verified locally | 本地验证通过
• No existing features affected | 无影响现有功能

Additional Notes | 其他说明

Verified with npm test and npm run build using Node v24.15.0.

[Maxwell-Code07]

Thanks for the PR! We'll review the platform-aware config discovery approach.

[YOMXXX]

Triage note: #443 is now the canonical verified fix for #103.

I previously could not select #221 because the local focused tests still resolved the default port instead of the test fixture config path. #443 adds shared config-path helpers, covers XDG/HOME/USERPROFILE fallback behavior, and passed local verification:

• COREPACK_ENABLE_AUTO_PIN=0 pnpm vitest run src/utils/config-paths.test.ts
• COREPACK_ENABLE_AUTO_PIN=0 pnpm test
• COREPACK_ENABLE_AUTO_PIN=0 pnpm build
• git diff --check

Recommendation: close this as superseded by #443, unless there is a small behavior difference that should be split into a follow-up.

[RerankerGuo]

Closing as superseded by #443 based on the triage note. #443 is now the verified canonical fix for #103 with shared config-path helpers and broader XDG/HOME/USERPROFILE fallback coverage, so this older branch should not keep consuming review attention.