ci: route gates to general and AI runners

[Daliys]

Summary

Aligns NexusUnity CI with the shared runner contract and protects both Ollama-based checks from concurrent execution.

Changes

• Routes Code Linter, Python Quality Gate, and Unity Quality Gate to Default + ci-scope.
• Routes Slop Review to Default + ci-scope-ai with Ollama model qwen3-coder:30b-a3b-q4_K_M.
• Routes the in-repository Documentation quality AI job to ci-scope-ai.
• Gives the two AI jobs the shared non-cancelling ci-scope-slop-review concurrency group.

Why

The previous workflows targeted the legacy ci-scope-v2 label, and the documentation AI job used the general runner. The updated contract keeps ordinary jobs parallel while ensuring AI jobs use the single RAM-heavy runner.

The V2 canary workflow remains unchanged because it is an isolated controlled canary path.

Validation

• Workflow YAML parsed successfully.
• git diff --check
• All required NexusUnity CI checks will be verified before merge.