fix(gateway): default Slack tool_progress to off

Slack Bolt posts are not editable like CLI spinners; medium-tier new still emitted a permanent line per tool start (issue #14663).

- Built-in slack default: off; other tier-2 platforms unchanged.

- Adjust /verbose isolation test for off to new cycle.

- Migration tests: read/write config.yaml as UTF-8 (Windows locale).

gateway/display_config.py

@@ -79,7 +79,9 @@ _PLATFORM_DEFAULTS: dict[str, dict[str, Any]] = {
     "discord":     _TIER_HIGH,
 
     # Tier 2 — edit support, often customer/workspace channels
-    "slack":           _TIER_MEDIUM,
+    # Slack: tool_progress off by default — Bolt posts cannot be edited like CLI;
+    # "new"/"all" spam permanent lines in channels (hermes-agent#14663).
+    "slack":           {**_TIER_MEDIUM, "tool_progress": "off"},
     "mattermost":      _TIER_MEDIUM,
     "matrix":          _TIER_MEDIUM,
     "feishu":          _TIER_MEDIUM,