[task] Define shared GitHub workflow data types

[phongndo]

Goal

Define the first shared Rust and protocol-facing data shapes for GitHub workflow state.

Why this matters

The TUI and desktop should render the same issue, PR, checks, and review concepts. Shared types keep frontend surfaces from inventing different meanings for the same GitHub data.

Command/API target

Document and implement initial behavior-level types for repository identity, issue summary/detail, PR summary/detail, check summary, and review thread/comment summary.

Depends on

• [task] Add GitHub API adapter using gh api #4 for GitHub adapter direction.
• [task] Resolve current GitHub repository #5 for current repository identity.

Out of scope

• Do not freeze every field as a long-term public protocol contract.
• Do not add GraphQL pagination complexity beyond what later issues need.
• Do not build UI rendering here.

Definition of done

• Shared data shapes exist for the first app-server GitHub methods.
• Type names and field purposes are clear enough for TUI and desktop contributors.
• The docs or code comments state that exact schemas may evolve during pre-alpha.

Verification

• cargo fmt --all --check
• cargo check --workspace
• TypeScript protocol generation still works if touched.

Contributor notes

This is maintainer-owned because it defines the common language for later issues.