fix: keep prompt caret stable during autosave (#57)

[blueberrycongee]

The prompt TextEditor was bound directly to a featureSettings field whose parent reassigns the entire FeatureSettings struct on every debounced save (saveFeatureSettings -> reloadFeatureSettings). SwiftUI then re-stringifies the underlying NSTextView, which collapses the caret to the end of the document mid-typing.

Decouple the editor from the persisted binding: FeaturePromptSection now owns a local draft @State, the TextEditor is bound to the draft, and we push the draft into the parent binding (and call persistChanges) only at flush time. External mutations to the bound text are still pulled into the draft, but round-trip echoes of our own writes are ignored, so the NSTextView never gets a redundant string assignment while the user is typing.

[hehehai]

补充说明一下这次在 PR 上追加的修改：

1. 对 FeaturePromptSection 做了状态协调优化：把编辑中的 draft、外部同步值、以及 debounce flush 的判定集中到 FeaturePromptDraftCoordinator，避免 autosave / reload 时状态分散，方便后续维护和回归。
2. 新增了 FeaturePromptDraftCoordinatorTests，覆盖初始化、正常 flush、stale debounce 忽略、忽略自身 round-trip echo、以及外部真实更新同步这几个关键场景，补齐这次光标保持修复的回归测试。
3. 修复了 VoxtTests target 的签名配置漂移问题：之前 test bundle 被 ad-hoc 签名，导致在本机运行测试时因为 Team ID 不一致无法加载。现在已经对齐到和宿主 app 一致的开发签名，并补上 test bundle 的 Info.plist 生成配置。

本地验证结果：

• xcodebuild build-for-testing -project Voxt.xcodeproj -scheme Voxt -configuration TestDebug -destination "platform=macOS" 通过
• xcodebuild test-without-building -project Voxt.xcodeproj -scheme Voxt -configuration TestDebug -destination "platform=macOS" -derivedDataPath /Users/guanwei/x/doit/Voxt/build/DerivedDataFeaturePromptFixed -only-testing:VoxtTests/FeaturePromptDraftCoordinatorTests 通过（6/6）

这几处改动的目标是把这次提示词 autosave 修复补成一组可维护、可回归验证的改动，同时恢复本地测试体验。