feat(tui): per-section visibility for the details accordion

Adds optional per-section overrides on top of the existing global
details_mode (hidden | collapsed | expanded).  Lets users keep the
accordion collapsed by default while auto-expanding tools, or hide the
activity panel entirely without touching thinking/tools/subagents.

Config (~/.hermes/config.yaml):

    display:
      details_mode: collapsed
      sections:
        thinking: expanded
        tools:    expanded
        activity: hidden

Slash command:

  /details                              show current global + overrides
  /details [hidden|collapsed|expanded]  set global mode (existing)
  /details <section> <mode|reset>       per-section override (new)
  /details <section> reset              clear override

Sections: thinking, tools, subagents, activity.

Implementation:

- ui-tui/src/types.ts             SectionName + SectionVisibility
- ui-tui/src/domain/details.ts    parseSectionMode / resolveSections /
                                  sectionMode + SECTION_NAMES
- ui-tui/src/app/uiStore.ts +
  app/interfaces.ts +
  app/useConfigSync.ts            sections threaded into UiState
- ui-tui/src/components/
  thinking.tsx                    ToolTrail consults per-section mode for
                                  hidden/expanded behaviour; expandAll
                                  skips hidden sections; floating-alert
                                  fallback respects activity:hidden
- ui-tui/src/components/
  messageLine.tsx + appLayout.tsx pass sections through render tree
- ui-tui/src/app/slash/
  commands/core.ts                /details <section> <mode|reset> syntax
- tui_gateway/server.py           config.set details_mode.<section>
                                  writes to display.sections.<section>
                                  (empty value clears the override)
- website/docs/user-guide/tui.md  documented

Tests: 14 new (4 domain, 4 useConfigSync, 3 slash, 3 gateway).
Total: 269/269 vitest, all gateway tests pass.

tests/test_tui_gateway_server.py

@@ -160,6 +160,71 @@ def test_config_set_statusbar_survives_non_dict_display(tmp_path, monkeypatch):
     assert saved["display"]["tui_statusbar"] == "bottom"
 
 
+def test_config_set_section_writes_per_section_override(tmp_path, monkeypatch):
+    import yaml
+
+    cfg_path = tmp_path / "config.yaml"
+    monkeypatch.setattr(server, "_hermes_home", tmp_path)
+
+    resp = server.handle_request(
+        {
+            "id": "1",
+            "method": "config.set",
+            "params": {"key": "details_mode.activity", "value": "hidden"},
+        }
+    )
+
+    assert resp["result"] == {"key": "details_mode.activity", "value": "hidden"}
+    saved = yaml.safe_load(cfg_path.read_text())
+    assert saved["display"]["sections"] == {"activity": "hidden"}
+
+
+def test_config_set_section_clears_override_on_empty_value(tmp_path, monkeypatch):
+    import yaml
+
+    cfg_path = tmp_path / "config.yaml"
+    cfg_path.write_text(
+        yaml.safe_dump(
+            {"display": {"sections": {"activity": "hidden", "tools": "expanded"}}}
+        )
+    )
+    monkeypatch.setattr(server, "_hermes_home", tmp_path)
+
+    resp = server.handle_request(
+        {
+            "id": "1",
+            "method": "config.set",
+            "params": {"key": "details_mode.activity", "value": ""},
+        }
+    )
+
+    assert resp["result"] == {"key": "details_mode.activity", "value": ""}
+    saved = yaml.safe_load(cfg_path.read_text())
+    assert saved["display"]["sections"] == {"tools": "expanded"}
+
+
+def test_config_set_section_rejects_unknown_section_or_mode(tmp_path, monkeypatch):
+    monkeypatch.setattr(server, "_hermes_home", tmp_path)
+
+    bad_section = server.handle_request(
+        {
+            "id": "1",
+            "method": "config.set",
+            "params": {"key": "details_mode.bogus", "value": "hidden"},
+        }
+    )
+    assert bad_section["error"]["code"] == 4002
+
+    bad_mode = server.handle_request(
+        {
+            "id": "2",
+            "method": "config.set",
+            "params": {"key": "details_mode.tools", "value": "maximised"},
+        }
+    )
+    assert bad_mode["error"]["code"] == 4002
+
+
 def test_enable_gateway_prompts_sets_gateway_env(monkeypatch):
     monkeypatch.delenv("HERMES_EXEC_ASK", raising=False)
     monkeypatch.delenv("HERMES_GATEWAY_SESSION", raising=False)