fix(stack): write JSON output to stdout instead of stderr (#1191)

`stack list --json` used `console.print()` which writes to stderr via
Rich. Switch to `click.echo()` so JSON goes to stdout and can be
piped to `jq` or other tools, matching queue and freeze commands.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Depends-On: #1190

Author: jd

mergify_cli/stack/list.py

@@ -21,6 +21,8 @@
 import sys
 import typing
 
+import click
+
 from mergify_cli import console
 from mergify_cli import utils
 from mergify_cli.exit_codes import ExitCode
@@ -479,6 +481,6 @@ async def stack_list(
     )
 
     if output_json:
-        console.print(json.dumps(output.to_dict(), indent=2))
+        click.echo(json.dumps(output.to_dict(), indent=2))
     else:
         display_stack_list(output, verbose=verbose)