Pilot coordinator delegation in issue execution

[vitormattos]

Summary

Extend the existing execute-issue skill into the smallest useful coordinator for LibreSign's orchestration pilot.

Instead of adding a new framework, permanent role agents, WORKFLOW.md or SPEC.md, this change:

• keeps single-flow execution as the default;
• allows temporary delegation only for genuinely independent investigation;
• keeps repository writes owned by the coordinator by default;
• requires isolated worktrees/runtimes before delegated writes;
• defines a concrete hosted vs self-hosted decision rule;
• adds evaluation cases for good and bad delegation;
• records the metrics needed to compare single-flow and coordinated runs.

This is intended as the first implementation increment for LibreSign/libresign#8989.

Validation

• commit created with the Git Signing MCP;
• DCO trailer uses the configured identity;
• cryptographic signature verified;
• package structural CI will validate the branch after PR creation.

Scope

This PR deliberately does not add a new orchestration runtime. The current plugin/skill surface remains the versioned control plane, and orchestration is treated as behavior of execute-issue rather than a separate infrastructure layer.