[Task] Add an integration request Discussion template

[ryan-wong-coder]

Background\n\nGitHub Discussions is now the preferred place to explore integrations before creating engineering issues. A template will help users provide actionable context.\n\n## Scope\n\nAdd a GitHub Discussion form or template for integration proposals.\n\n## Acceptance criteria\n\n- Requests the external system, user workflow, deployment model, expected evidence, trust roots, scale, and verification experience.\n- Clearly separates implemented behavior from requested behavior.\n- Reminds users not to post credentials, keys, customer data, or private endpoints.\n- Links to the roadmap, security policy, and contribution guide.