Add issue templates and custom fields for different issue types #21

Problem

All issues look the same - title, description, status, priority. But teams need different workflows for bugs vs features vs tasks. A bug report needs reproduction steps and severity. A feature request needs user story format and business impact.

Expected

Customizable issue templates that pre-populate fields, and custom fields that can be defined per-team.

Acceptance Criteria

• Issue type selector: Bug, Feature, Task, Custom
• Each type has a default template (markdown body)
• Custom fields: text, number, select, date (stored as JSON in issues table)
• Templates configurable in team settings
• Free tier: 3 templates max. Pro: unlimited
• AI agent can use templates when creating issues