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

**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//DerivedData`). This is *separate* from Xcode.app's `~/Library/Developer/Xcode/DerivedData/-`, 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).

Start at resolveEffectiveDerivedDataPath and review the per-workspace isolation introduced by #340. Inspect how Xcode.app DerivedData folders expose WorkspacePath in info.plist, then define how matching and fallback paths should work. Done means each workspace can reuse its GUI DerivedData without collisions, while workspaces without an existing folder retain deterministic isolation.

Written by the indexing model from the issue text.