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.

ui-tui/src/__tests__/createSlashHandler.test.ts

@@ -88,6 +88,41 @@ describe('createSlashHandler', () => {
     expect(ctx.transcript.sys).toHaveBeenCalledWith('details: expanded')
   })
 
+  it('sets a per-section override and persists it under details_mode.<section>', () => {
+    const ctx = buildCtx()
+
+    expect(createSlashHandler(ctx)('/details activity hidden')).toBe(true)
+    expect(getUiState().sections.activity).toBe('hidden')
+    expect(ctx.gateway.rpc).toHaveBeenCalledWith('config.set', {
+      key: 'details_mode.activity',
+      value: 'hidden'
+    })
+    expect(ctx.transcript.sys).toHaveBeenCalledWith('details activity: hidden')
+  })
+
+  it('clears a per-section override on /details <section> reset', () => {
+    const ctx = buildCtx()
+    createSlashHandler(ctx)('/details tools expanded')
+    expect(getUiState().sections.tools).toBe('expanded')
+
+    createSlashHandler(ctx)('/details tools reset')
+    expect(getUiState().sections.tools).toBeUndefined()
+    expect(ctx.gateway.rpc).toHaveBeenLastCalledWith('config.set', {
+      key: 'details_mode.tools',
+      value: ''
+    })
+    expect(ctx.transcript.sys).toHaveBeenCalledWith('details tools: reset')
+  })
+
+  it('rejects unknown section modes with a usage hint', () => {
+    const ctx = buildCtx()
+    createSlashHandler(ctx)('/details tools blink')
+    expect(getUiState().sections.tools).toBeUndefined()
+    expect(ctx.transcript.sys).toHaveBeenCalledWith(
+      'usage: /details <section> [hidden|collapsed|expanded|reset]'
+    )
+  })
+
   it('shows tool enable usage when names are missing', () => {
     const ctx = buildCtx()