Design Agentree work-node schema and Gira identity mapping

[StatPan]

Goal

Write the first design note for Agentree's Gira-based work map model.

Scope

• Define the work-node schema.
• Define the common identity key: repo + branch + Gira ticket.
• Specify how opencode session metadata maps onto work nodes.
• Specify how GitHub PR/check evidence maps onto work nodes.
• List unknowns for Codex/Claude read-only adapters.

Acceptance Criteria

• A repo doc exists under docs/ describing the schema and mapping.
• The doc references [Epic] Gira-based agent work map roadmap #15 as the parent roadmap epic.
• The doc keeps v1 opencode-native and does not require universal harness control.

Parent

Part of #15.