chore: migrate from bun to pnpm + vitest + esbuild

- Replace bun:test with vitest across all packages
- Replace bun build with esbuild
- Replace bun:sqlite with better-sqlite3
- Fix OCAS Store API: store.put/get → store.cas.put/get
- Fix vitest vi.mock hoisting (vi.hoisted)
- Add pnpm-workspace.yaml and pnpm-lock.yaml
- Update all package.json test/build scripts

WIP: 8 failures remain in agent-hermes (bun engines check + sqlite migration)

Refs #26

packages/agent-hermes/__tests__/session-detail.test.ts

@@ -1,5 +1,5 @@
-import { Database } from "bun:sqlite";
-import { describe, expect, test } from "bun:test";
+import Database from "better-sqlite3";
+import { describe, expect, test } from 'vitest';
 import { mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -106,7 +106,7 @@ describe("storeHermesSessionDetail", () => {
 
     expect(output).toBe("done");
 
-    const detailNode = store.get(detailHash);
+    const detailNode = store.cas.get(detailHash);
     expect(detailNode).not.toBeNull();
     if (detailNode === null) {
       return;