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/cli/src/commands/setup.ts

@@ -175,13 +175,11 @@ export async function _discoverAgents(): Promise<string[]> {
 
 async function _tryWhichDiscovery(): Promise<string[] | null> {
   try {
-    const proc = Bun.spawn(["which", "-a", "uwf-hermes", "uwf-claude-code", "uwf-cursor"], {
-      stdout: "pipe",
-      stderr: "pipe",
+    const { execFileSync } = await import("node:child_process");
+    const text = execFileSync("which", ["-a", "uwf-hermes", "uwf-claude-code", "uwf-cursor"], {
+      encoding: "utf-8",
+      stdio: ["pipe", "pipe", "pipe"],
     });
-    const text = await new Response(proc.stdout).text();
-    await proc.exited;
-    if (proc.exitCode !== 0) return null;
     return _parseWhichOutput(text);
   } catch {
     return null;