feat: add standalone ticket implementation skill

[shaug]

TL;DR

Adds implement-ticket as the reusable, runtime-neutral workflow for implementing exactly one standalone ticket or named epic child through safe review and delivery gates.

Summary

• Extract the single-ticket readiness, implementation, validation, review, PR, merge, transition, and cleanup contract from epic orchestration.
• Establish implement-ticket as the canonical owner of generic one-ticket behavior while preserving temporary compatibility with implement-epic-sequence until Replace implement-epic-sequence with implement-epic as an implement-ticket orchestrator #19.
• Keep the dependency graph one-way: implement-epic may consume implement-ticket, while whole-epic requests return a pre-mutation requires_epic handoff with cycle protection.
• Preserve cross-system GitHub and Linear ownership, explicit authority boundaries, current-candidate review gates, safe base-drift handling, and post-merge dependency refresh.
• Add forward scenarios and contract checks for standalone tickets, epic children, missing prerequisites, duplicate ownership, review cycles, tracker/PR-host splits, and epic routing.

The Fixes #18 reference below means an authorized future merge will automatically close issue #18. This PR does not grant or exercise merge authority.

Validation

• just format
• just test-implement-ticket
• just test
• just lint
• git diff --check
• Fresh-context forward evaluation across the required ticket, review, tracker, and epic-routing scenarios
• Repository-owned review-code-change: clean aggregate on 7113afd5ab04d0200c2bfa6b5008d9fcd2b2f7f6

Tickets

Fixes #18