fix(commands): resolve --assignee to UUID in issues create/update

[iamfj]

What

Fix for --assignee option on issues create and issues update failing with a GraphQL validation error when given a human-friendly name or email instead of a UUID.

Problem

The --assignee value was passed directly to input.assigneeId without resolution. Every other identifier option (--team, --project, --labels, --cycle, --status, --parent-ticket, --project-milestone) goes through a dedicated resolver, but --assignee was missing one, causing:

{"error": "GraphQL request failed: Argument Validation Error - assigneeId must be a UUID."}

Solution

• Added resolveUserId() resolver (src/resolvers/user-resolver.ts) using LinearSdkClient, following the established resolver pattern
• Lookup order: UUID passthrough → display name (case-insensitive) → email (case-insensitive)
• Handles multipleMatchesError when name is ambiguous, notFoundError when no match exists
• Wired resolver into both issues create and issues update commands

Testing

• 5 resolver unit tests: UUID passthrough, name lookup, email fallback, not-found error, multiple-matches error
• 5 command-level integration tests: verify resolveUserId is called with correct args and its UUID flows into createIssue/updateIssue for both name and email inputs, plus omitted-assignee cases
• All 143 tests passing, build clean

Checklist

• Bug resolved
• Regression tests added
• No side effects introduced

Closes #47