test(sessions): resolve POSIX fixture paths that are drive-relative on Windows

Five tests failed only on the Windows runner. Their fixtures used bare
POSIX-absolute paths such as /srv/other-worktree and /old-project, which
win32 treats as absolute but drive-relative: resolve() maps them onto the
runner's current drive (D:\...). The code under test canonicalizes stored
cwds by contract, so assertions comparing against the raw fixture string
never matched on Windows:

- parentSessionLocator and crossWorkspace listings annotate
  parentSessionCwd with canonicalizeStoredCwd(header.cwd);
- cleanup forgets unread via canonicalizeStoredCwd(record.cwd), which
  missed the marker the test seeded with the raw path.

Resolve the fixture paths once at declaration, matching the existing
WORKSPACE_CWD = resolve("/workspace") convention, so fixtures model what
a real Windows Pi would record. Linux behavior is unchanged.

src/server/sessions/piSessionService.crossWorkspace.test.ts

@@ -1,13 +1,16 @@
 import { mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
-import { join } from "node:path";
+import { join, resolve } from "node:path";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { PiSessionService, type PiSessionListEntry } from "./piSessionService.js";
 import { CapturingSessionEventHub, emptyArchiveStore, fakeSessionManager, sessionRecord, testModelRuntime, type SessionGateway } from "./piSessionService.testSupport.js";
 
 const TEST_AGENT_DIR = "/tmp/pi-web-test-agent";
 const CHILD_CWD = "/srv/dev/pi-web";
-const PARENT_CWD = "/srv/dev/pi-web-feature";
+// Resolved because the service canonicalizes header cwds before annotating: a
+// bare "/srv/..." is drive-relative on Windows and would land on the runner's
+// current drive.
+const PARENT_CWD = resolve("/srv/dev/pi-web-feature");
 
 let tempDir: string;