feat(session): auto-scope DerivedData per workspace/project path

When derivedDataPath is not explicitly set but workspacePath or
projectPath is known, the session store computes a workspace-scoped
subdirectory under DerivedData using a name+hash scheme (e.g.
MyApp-4ee6552f04c9). This prevents concurrent sessions building
different clones of the same project from colliding on a shared
build directory.

Closes #340

Author: codeman9

src/mcp/tools/session-management/__tests__/session_clear_defaults.test.ts

@@ -44,7 +44,8 @@ describe('session-clear-defaults tool', () => {
       const current = sessionStore.getAll();
       expect(current.scheme).toBeUndefined();
       expect(current.deviceId).toBeUndefined();
-      expect(current.derivedDataPath).toBeUndefined();
+      // derivedDataPath is computed from projectPath when not explicitly set
+      expect(current.derivedDataPath).toContain('proj-');
       expect(current.projectPath).toBe('/path/to/proj.xcodeproj');
       expect(current.simulatorName).toBe('iPhone 17');
       expect(current.useLatestOS).toBe(true);

src/utils/__tests__/session-store.test.ts

@@ -121,4 +121,38 @@ describe('SessionStore', () => {
     const stored = sessionStore.getAll();
     expect(stored.env).toEqual({ API_KEY: 'secret' });
   });
+
+  it('computes a workspace-scoped derivedDataPath when workspacePath is set', () => {
+    sessionStore.setDefaults({ workspacePath: '/Users/dev/clone-1/MyApp.xcworkspace' });
+
+    const defaults = sessionStore.getAll();
+    expect(defaults.derivedDataPath).toMatch(/MyApp-[a-f0-9]{12}$/);
+  });
+
+  it('computes a project-scoped derivedDataPath when projectPath is set', () => {
+    sessionStore.setDefaults({ projectPath: '/Users/dev/clone-2/MyApp.xcodeproj' });
+
+    const defaults = sessionStore.getAll();
+    expect(defaults.derivedDataPath).toMatch(/MyApp-[a-f0-9]{12}$/);
+  });
+
+  it('does not override an explicitly set derivedDataPath', () => {
+    sessionStore.setDefaults({
+      workspacePath: '/Users/dev/clone-1/MyApp.xcworkspace',
+      derivedDataPath: '/custom/path',
+    });
+
+    expect(sessionStore.getAll().derivedDataPath).toBe('/custom/path');
+  });
+
+  it('produces different hashes for different workspace paths', () => {
+    sessionStore.setDefaults({ workspacePath: '/clone-1/MyApp.xcworkspace' });
+    const path1 = sessionStore.getAll().derivedDataPath;
+
+    sessionStore.clearAll();
+    sessionStore.setDefaults({ workspacePath: '/clone-2/MyApp.xcworkspace' });
+    const path2 = sessionStore.getAll().derivedDataPath;
+
+    expect(path1).not.toBe(path2);
+  });
 });

src/utils/session-store.ts

@@ -1,3 +1,6 @@
+import * as crypto from 'node:crypto';
+import * as path from 'node:path';
+import { DERIVED_DATA_DIR } from './log-paths.ts';
 import { log } from './logger.ts';
 
 export type SessionDefaults = {
@@ -133,7 +136,19 @@ class SessionStore {
 
   getAllForProfile(profile: string | null): SessionDefaults {
     const defaults = profile === null ? this.globalDefaults : (this.profiles[profile] ?? {});
-    return this.cloneDefaults(defaults);
+    const result = this.cloneDefaults(defaults);
+
+    if (!result.derivedDataPath) {
+      const anchor = result.workspacePath ?? result.projectPath;
+      if (anchor) {
+        const resolved = path.resolve(anchor);
+        const hash = crypto.createHash('sha256').update(resolved).digest('hex').slice(0, 12);
+        const name = path.basename(resolved, path.extname(resolved));
+        result.derivedDataPath = path.join(DERIVED_DATA_DIR, `${name}-${hash}`);
+      }
+    }
+
+    return result;
   }
 
   listProfiles(): string[] {