Add Agent-native issue and PR sizing rules

[adohe]

Spec candidate: #7.

Scope

• Turn the sizing rules in the v0.0.3 spec into reusable contributor guidance.
• Require narrow review surfaces, explicit verification, and source references for agent-generated changes.
• Add examples of acceptable small issue shapes.

Acceptance

• Compatible with the existing AGENTS guide.
• Does not require new tooling.
• Explains when a task should be split into several issues.