feat: add support for user-type custom fields

[alastori]

Adds handling for user-type custom fields in both create and edit paths.

Problem

User-type custom fields passed via --custom are silently dropped. The CLI reports "Issue updated" but the field remains null. This happens because the custom field switch statement only handles option, array, number, project, and the default (string) cases — but Jira's REST API requires user fields to be objects ({"accountId":"..."} for Cloud, {"name":"..."} for Server/DC), not plain strings.

Solution

• Add customFieldTypeUser and customFieldTypeUserSet types following the existing pattern
• Add newCustomFieldTypeUser() constructor that branches on Cloud vs Local installation
• Handle user schema type for single-user fields in both constructCustomFields (create) and constructCustomFieldsForEdit (edit)
• Handle user items type for multi-user picker arrays in both paths
• Pass installationType through to constructCustomFields in the create path (edit already had it)
• Convert if/else chains to switch in the array sub-cases (gocritic lint)

How to test?

Unit tests (18 new tests):

go test -race ./pkg/jira/... -run "Custom|Edit|User" -v

Manual verification:

1. Create a free Jira Cloud instance at atlassian.com
2. Add a "User Picker (single user)" custom field (e.g., "PM owner")
3. Run jira init and map the field in config under issue.fields.custom with datatype: user
4. Create or edit an issue: jira issue edit TEST-1 --custom "pm-owner=<accountId>" --no-input
5. Verify the field is set via the Jira UI or REST API

Tested end-to-end on a real Jira Cloud instance.

Checklist

• Tests added
• Manual verification done
• Backwards compatible (all changed functions are unexported)
• Lint clean (golangci-lint 0 issues)

Fixes #798, #758, #579