Add draft_issue content_type support to update_project schema and validation

[Copilot]

The update_project handler's JavaScript implementation supports creating draft issues in Projects v2 via content_type: "draft_issue", but the schema and Go validation were missing this option.

Changes

• schemas/agent-output.json: Added "draft_issue" to content_type enum and added draft_title/draft_body properties
• pkg/workflow/safe_output_validation_config.go: Added "draft_issue" enum value and validation rules for draft_title (max 256) and draft_body (max 65000)

Usage

{
  "type": "update_project",
  "project": "https://github.com/orgs/myorg/projects/42",
  "content_type": "draft_issue",
  "draft_title": "Draft Item Title",
  "draft_body": "Optional markdown content",
  "fields": {"Status": "Todo", "Priority": "High"}
}

The JavaScript implementation in update_project.cjs already handles this fully (including 6 test cases). This change completes the type system coverage.

Original prompt

Update your project handler/action to support content_type "draft_issue".

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