Add assignGithubIssue helper

[SorraTheOrc]

Add assignGithubIssue helper to src/github.ts

Add new async function assignGithubIssueAsync (and sync assignGithubIssue) to src/github.ts that wraps gh issue edit <number> --add-assignee <user> with the existing retry/backoff infrastructure.

User Story

As a developer building the delegate command, I need a reusable helper function to assign a GitHub issue to a user so that the delegate command can call it without reimplementing the gh CLI wrapper logic.

Acceptance Criteria

1. assignGithubIssueAsync(config, issueNumber, assignee) calls gh issue edit <N> --add-assignee <user> and returns { ok: boolean; error?: string }.
2. Sync variant assignGithubIssue(config, issueNumber, assignee) exists for non-async callers.
3. Rate-limit retry/backoff logic is reused from existing runGhDetailedAsync.
4. If the gh command fails (exit code != 0), returns { ok: false, error: <stderr> } without throwing.
5. Unit tests with mocked gh calls verify success, failure, and retry scenarios.
6. Both functions are exported from src/github.ts for use by other modules.
7. If gh is not authenticated or unavailable, the function returns { ok: false, error: ... } without throwing.

Minimal Implementation

• Add assignGithubIssueAsync and assignGithubIssue functions to src/github.ts.
• Use runGhDetailedAsync / runGhDetailed internally.
• Add unit tests following existing patterns.

Dependencies

None (foundational layer).

Deliverables

• Updated src/github.ts
• New test file for assign helper

Key Files

• src/github.ts (lines 35-163 for existing runGh/runGhDetailed/runGhAsync patterns)

[SorraTheOrc]

\n\n**opencode**\n\nImplementation complete. PR #782 created (https://github.com/rgardler-msft/Worklog/pull/782). Branch: wl-0mm8lwwcd014htgu-assign-github-issue-helper, commit f8a6a4a. All 11 tests passing, full suite green (1159 tests). Awaiting review and merge.

[SorraTheOrc]

\n\n**worklog**\n\nClosed with reason: PR #782 and #783 merged. assignGithubIssue helper fully implemented and tested.