Allow reusing Xcode.app's per-workspace DerivedData (avoid duplicate builds); static XCODEBUILDMCP_DERIVED_DATA_PATH can't map per workspace

Bắt đầu tại resolveEffectiveDerivedDataPath và xem xét cơ chế cô lập theo workspace được giới thiệu bởi #340. Kiểm tra cách các thư mục DerivedData của Xcode.app cung cấp WorkspacePath trong info.plist, sau đó xác định cách các đường dẫn khớp và fallback cần hoạt động. Hoàn thành khi mỗi workspace có thể tái sử dụng DerivedData của GUI mà không xảy ra xung đột, còn các workspace chưa có thư mục hiện tại vẫn duy trì cơ chế cô lập xác định.

Version: 2.7.0

Problem

For derived data the MCP currently offers two modes, neither of which lets agent builds share Xcode.app's DerivedData per workspace:

1. Default: per-workspace isolation (#340), but under the MCP's own tree (~/Library/Developer/XcodeBuildMCP/workspaces/<workspace>/DerivedData). This is separate from Xcode.app's ~/Library/Developer/Xcode/DerivedData/<Workspace>-<hash>, so a project built both in the GUI and via the MCP is built and stored twice — double the disk and no incremental reuse across the two.
2. XCODEBUILDMCP_DERIVED_DATA_PATH: a single static path applied to every call (resolveEffectiveDerivedDataPath returns it verbatim). With multiple workspaces/worktrees, they all collide in one folder → build-product clobbering and full rebuilds.

Request

A mode to resolve derived data to the existing Xcode.app DerivedData folder for each workspace (e.g. matching WorkspacePath in each folder's info.plist), and/or per-workspace configurability instead of a single static override.

This would let a repo built in both Xcode and the MCP share one DerivedData per workspace — no duplication, true incremental reuse between interactive and agent builds.

Workaround

I currently achieve this with a client-side PreToolUse hook that rewrites each build/test call's derivedDataPath to the matching GUI folder (looked up via WorkspacePath in each folder's info.plist), falling back to a deterministic per-workspace path when no GUI folder exists yet. Happy to share it if useful.

Related

• #340 (per-workspace DerivedData isolation) — implemented isolation, but into the MCP's own tree rather than Xcode.app's.
• #283 (tilde expansion in config'd derived data path).