Fix API embedding fallback order and cache isolation

[RaviTharuma]

Summary

This fixes two related embedding issues:

• MEMORIX_EMBEDDING=auto now prefers a configured remote embedding API before falling back to local providers
• API embedding caches are now isolated by embedding config, including requested dimensions

Closes #25.

What changed

• added API-first resolution in auto mode when embedding API config is present
• namespaced API embedding cache keys by baseUrl + model + requestedDimensions
• namespaced probe-dimension cache entries by the same identity
• kept backward-compatible reads for the previous probe-dimension cache file shape
• added regression tests for API-first auto mode, cache isolation, and requested-dimension probe isolation
• documented the behavior change in docs/CONFIGURATION.md and CHANGELOG.md

Verification

Passed:

• npx vitest run tests/embedding/provider.test.ts tests/embedding/api-provider.test.ts
• npm run lint
• npm run build
• npx vitest run tests/integration/release-blockers.test.ts tests/git/extractor.test.ts tests/git/subdir-scan.test.ts

Note on remaining local failures:

• tests/git/extractor.test.ts
• tests/git/subdir-scan.test.ts

Those still fail in my local fork because the assertions are hard-coded to the upstream remote (AVIDS2/memorix) and to a specific sibling-repo layout, while this branch is running from RaviTharuma/memorix inside a workspace that also contains CLIProxyAPI. They are unrelated to this patch.

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[AVIDS2]

我手动对照了当前 main 和这次修复的目标，两个核心问题在主线里都是真实存在的：

1. MEMORIX_EMBEDDING=auto 目前仍然优先本地 fastembed / transformers，不会在 API embedding 已配置时优先走远端 API。
2. API embedding cache 与 probe-dimension cache 目前没有按完整 embedding config（尤其是 requestedDimensions）隔离，存在 stale cache 复用风险。

这条 PR 的修复范围集中、方向正确，而且把测试和文档一起补上了：

• auto 改为 API-first（当 embedding API 配置存在时）
• embedding cache 与 probe-dimension cache 按 baseUrl + model + requestedDimensions 隔离
• 保留旧 probe cache 结构的兼容读取
• 增加了对应回归测试

我已经手动复审过，批准这条 PR。感谢这次补得很扎实。