feat: add --json flag to issue list, issue create, and cycle list

[mihai-chiorean]

Summary

Adds --json / -j flag to three commands that were missing structured output support:

• issue list --json — outputs issues as a JSON array with id, identifier, title, priority, estimate, state, labels, assignee, and updatedAt
• issue create --json — outputs the created issue's id, identifier, url, and team as JSON (non-interactive path only)
• cycle list --json — outputs cycles as a JSON array with id, number, name, startsAt, endsAt, completedAt, isActive, isFuture, isPast, and status

When --json is set, the spinner is suppressed and empty results output [] instead of human-readable messages.

This is particularly useful for AI agents and automation that need to parse CLI output programmatically. The linear api raw GraphQL command works as a workaround today, but first-class --json support on list/create commands is much more ergonomic.

Related: #127 (requests --json on project commands — this PR covers issue and cycle commands using the same pattern)

Test plan

• Added snapshot test for issue list --json with mock server
• Added snapshot test for issue create --json with mock server
• Added snapshot test for cycle list --json with mock server
• Updated help text snapshots for all three commands
• deno fmt clean
• deno lint clean
• All new tests pass, existing tests unaffected