fix(linear): use project_id when creating issues via sync --push (GH#973) (#1012)

The linear.project_id config was being read and used for filtering
when pulling issues from Linear, but was not being passed when
creating new issues via --push. Now CreateIssue includes projectId
in the mutation input when configured.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/linear/client.go

@@ -418,6 +418,11 @@ func (c *Client) CreateIssue(ctx context.Context, title, description string, pri
 		"description": description,
 	}
 
+	// Include project if configured
+	if c.ProjectID != "" {
+		input["projectId"] = c.ProjectID
+	}
+
 	if priority > 0 {
 		input["priority"] = priority
 	}