Add GitHub Ticket port for list, Claim, Done, and Abort

[maxBRT]

Summary

• Adds a fakeable ticket.Port (ListReady / Claim / Done / Abort) so the Run loop can drive GitHub-backed Tickets without owning tracker details
• Implements ticket.GitHub via an injectable gh Exec seam: GraphQL list with Priority-then-oldest ordering and optional feature-label filter; Claim/Abort flip ready-for-agent ↔ in-progress; Done closes the issue
• Covers ordering, feature filter, and state transitions with recorded gh responses at that seam (closes GitHub Ticket port: list, Claim, and Done #7)

Test plan

• go test ./...
• Confirm ListReady orders High before Low, then oldest within a priority, with unset last
• Confirm --feature / feature arg keeps only matching Ready for Agent Tickets
• Confirm Claim/Abort label edits and Done issue close against a real repo when smoke-testing with live gh

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