refactor(events)!: simplify event identity and buffering

[laipz8200]

Important

1. Make sure you have read our contribution guidelines
2. Search existing issues and pull requests to confirm this change is not a duplicate
3. Open or identify the issue this pull request resolves or advances
4. Use a Conventional Commits title for this pull request, and mark breaking changes with !
5. Remember that the pull request title will become the squash merge commit message
6. If CLA Assistant prompts you, sign CLA.md in the pull request conversation

Related Issue

Closes #244

Summary

• Replace the append-only event list, custom read/write lock, and polling loop with a condition-backed deque that releases consumed events.
• Give graph events distinct event, graph, execution, and node-execution identities while preserving execution identity and sequence across runtime snapshots.
• Keep layer observation and consumer ordering deterministic for concurrent producers.
• Remove unused AgentNode event structures, exports, and metadata constants, and rely on native event models instead of a custom serialization helper.
• This changes the public event contract: workflow_id becomes graph_id, node correlation moves from id to node_execution_id, and AgentNode event APIs are removed.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation