Support editing GitHub issue assignees

[Harry19081]

Problem

GitHub issue detail views can display assignees, but users cannot edit them consistently from the chat, work-management, and workstation surfaces. The detail-state hook also lacks a shared assignee option model and mutation lifecycle.

Solution

Add a shared assignee normalizer and extend the GitHub issue detail state with assignee discovery, selection, optimistic mutation state, and error handling. Wire the resulting controls through the chat panel, work-management mutation bridge, workstation source-control views, and unified issue renderer so all issue-detail entry points share the same behavior.

Potential risks

Assignee updates still depend on repository permissions and GitHub API availability; failed mutations retain the server-backed value and surface the existing error path. The visual selector states have not been manually exercised in the desktop app, so this PR remains a draft pending UI evidence.

Verification

• pnpm exec vitest run src/modules/shared/hooks/useGitHubIssueDetailState.test.ts — passed (4 tests).
• Commit hook lint-staged — passed for all 8 changed files.
• Commit hook scoped TypeScript check — passed.
• Manual desktop verification was not run because local UI control was not authorized for this task.