feat: add bounded two-agent completion pipeline

[Quazmoz]

Summary

Adds the bounded two-agent VS Code/GitHub Copilot completion pipeline as a clean single commit on top of current main, while preserving the existing Codebase Maintenance and De-Slop stack.

Key properties:

• Qwen lead/reviewer roles without fabricated model IDs; manual model-picker fallback
• deterministic task state, findings, verification logs, and completion gate
• bounded iteration/failure/review/stop-hook limits
• stale-evidence detection via workspace fingerprints
• explicit approval provenance and escalation semantics
• real-artifact requirement for visual acceptance
• read-mostly adversarial reviewer adapter
• 15-scenario bounded-pipeline regression validator
• canonical validation suite includes both maintenance and bounded-completion validators

Validation

PR CI should run the repository canonical entry point:

python3 scripts/validate-agentdefaults.py

Do not merge if any validator fails.