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__/issue-551.test.ts

@@ -1,8 +1,10 @@
-import { describe, expect, test } from "bun:test";
+import { describe, expect, test } from 'vitest';
+import { dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
 import { readFileSync } from "node:fs";
 import { join } from "node:path";
 
-const PKG_ROOT = join(import.meta.dir, "..");
+const PKG_ROOT = join(dirname(fileURLToPath(import.meta.url)), "..");
 
 describe("Issue #551 — bin entry & engines", () => {
   test("package.json declares bun in engines", () => {