Prototype Gira/GitHub read model for ticket, branch, PR, and checks

[StatPan]

Goal

Prototype a read-only Gira/GitHub evidence layer that Agentree can use to enrich canvas nodes with ticket, branch, PR, and check state.

Scope

• Read repo/branch context from local git or opencode session metadata where available.
• Resolve Gira ticket identity from branch naming or GitHub issue/PR evidence.
• Resolve linked PR and check status.
• Expose a small internal read model for the frontend.

Acceptance Criteria

• Read-only behavior only; no GitHub mutation.
• Unit tests cover branch-to-ticket parsing and missing/ambiguous evidence.
• The frontend can display at least ticket number/title, branch, PR state, and check summary when available.

Parent

Part of #15.