feat(events): add graph event filters

[laipz8200]

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Related Issue

Closes #145

Summary

Adds canonical graph traversal events and an explicit graph event filter pipeline for output-time event shaping.

Moves response stream ordering and provider-facing output shaping into ResponseStreamFilter, while removing response coordinator state from the core runtime path.

Updates the slim LLM example to apply the response stream filter explicitly and keep CLI output plain.

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