test: add shared mock infrastructure and route tests (phase 7)

Consolidate duplicated MockSession/MockStateStore into test/mocks/,
migrate respawn tests to shared mocks, and add 58 route tests for
session, system, and respawn endpoints using Fastify app.inject().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/mocks/index.ts

@@ -0,0 +1,11 @@
+/**
+ * Shared test mocks — import from here instead of defining inline.
+ *
+ * @example
+ * import { MockSession, MockStateStore, terminalOutputs } from './mocks/index.js';
+ */
+
+export { MockSession, createMockSession, terminalOutputs } from './mock-session.js';
+export { MockStateStore } from './mock-state-store.js';
+export { waitForEvent, createDeferred } from './test-helpers.js';
+export { createMockRouteContext, type MockRouteContext } from './mock-route-context.js';