feat(tarko-agent): remove engine-selector from agent kernel

[ulivz]

Summary

1. Removed engine-selector from the Agent Kernel to simplify the architecture.
2. Migrate all examples and their snapshot tests from Seed-1.5 VL to Seed 1.6 VL

Checklist

• Added or updated necessary tests (Optional).
• Updated documentation to align with changes (Optional).
• Verified no breaking changes, or prepared solutions for any occurring breaking changes (Optional).
• My change does not involve the above items.

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