Add --json output to issue create

Context

The tangled issue create command currently only produces human-readable output. In scripting and LLM workflows it's common to capture the result of a create command and pass fields (like the issue number or URI) directly to a subsequent command without parsing formatted text.

All other read commands (issue list, issue view) already support --json, so issue create is an inconsistent gap.

Tasks

• Add --json [fields] option to tangled issue create, consistent with the existing --json pattern on issue list and issue view.
  • JSON output should include at minimum: number, title, uri, state, createdAt.
  • Support optional comma-separated field filtering (e.g. --json number,uri) to match existing behaviour.
• Add tests for JSON output from issue create, covering full output and field-filtered output.

Notes

Depends on (or should be done alongside) issue #8, which adds the sequential number to the human-readable output — the same resolved number should appear in the JSON output. Follows the field-filtering pattern already implemented in src/utils/formatting.ts.