ci(agent): verify release build before merge

[jonathanlab]

Problem

Agent release builds can diverge from the PR build and only fail after an automated release tag is created.

Changes

• extract the release build/test steps into a shared composite action
• run that exact action on relevant pull requests before merge
• build the complete agent workspace dependency graph instead of maintaining a manual list

How did you test this?

• pnpm install --frozen-lockfile
• pnpm --filter @posthog/agent... run build
• pnpm --filter @posthog/agent run test

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 2355694.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
.github/workflows/agent-release-verify.yml:16
**Workspace Config Skips Verification**

A change to `pnpm-workspace.yaml` can alter workspace catalogs, dependency overrides, or install policies used by the agent release build, but it does not trigger this workflow. Such a change can merge without running the release check and then fail when the tag workflow performs the same frozen install and build.

```suggestion
      - "pnpm-lock.yaml"
      - "pnpm-workspace.yaml"
```

_{Reviews (1): Last reviewed commit: "ci(agent): verify release build before m..." | Re-trigger Greptile}