feat(tui): subagent spawn observability overlay

Adds a live + post-hoc audit surface for recursive delegate_task fan-out.
None of cc/oc/oclaw tackle nested subagent trees inside an Ink overlay;
this ships a view-switched dashboard that handles arbitrary depth + width.

Python
- delegate_tool: every subagent event now carries subagent_id, parent_id,
  depth, model, tool_count; subagent.complete also ships input/output/
  reasoning tokens, cost, api_calls, files_read/files_written, and a
  tail of tool-call outputs
- delegate_tool: new subagent.spawn_requested event + _active_subagents
  registry so the overlay can kill a branch by id and pause new spawns
- tui_gateway: new RPCs delegation.status, delegation.pause,
  subagent.interrupt, spawn_tree.save/list/load (disk under
  \$HERMES_HOME/spawn-trees/<session>/<ts>.json)

TUI
- /agents overlay: full-width list mode (gantt strip + row picker) and
  Enter-to-drill full-width scrollable detail mode; inverse+amber
  selection, heat-coloured branch markers, wall-clock gantt with tick
  ruler, per-branch rollups
- Detail pane: collapsible accordions (Budget, Files, Tool calls, Output,
  Progress, Summary); open-state persists across agents + mode switches
  via a shared atom
- /replay [N|last|list|load <path>] for in-memory + disk history;
  /replay-diff <a> <b> for side-by-side tree comparison
- Status-bar SpawnHud warns as depth/concurrency approaches caps;
  overlay auto-follows the just-finished turn onto history[1]
- Theme: bump DARK dim #B8860B → #CC9B1F for readable secondary text
  globally; keep LIGHT untouched

Tests: +29 new subagentTree unit tests; 215/215 passing.

ui-tui/src/app/overlayStore.ts

@@ -3,6 +3,8 @@ import { atom, computed } from 'nanostores'
 import type { OverlayState } from './interfaces.js'
 
 const buildOverlayState = (): OverlayState => ({
+  agents: false,
+  agentsInitialHistoryIndex: 0,
   approval: null,
   clarify: null,
   confirm: null,
@@ -18,8 +20,8 @@ export const $overlayState = atom<OverlayState>(buildOverlayState())
 
 export const $isBlocked = computed(
   $overlayState,
-  ({ approval, clarify, confirm, modelPicker, pager, picker, secret, skillsHub, sudo }) =>
-    Boolean(approval || clarify || confirm || modelPicker || pager || picker || secret || skillsHub || sudo)
+  ({ agents, approval, clarify, confirm, modelPicker, pager, picker, secret, skillsHub, sudo }) =>
+    Boolean(agents || approval || clarify || confirm || modelPicker || pager || picker || secret || skillsHub || sudo)
 )
 
 export const getOverlayState = () => $overlayState.get()
@@ -27,4 +29,23 @@ export const getOverlayState = () => $overlayState.get()
 export const patchOverlayState = (next: Partial<OverlayState> | ((state: OverlayState) => OverlayState)) =>
   $overlayState.set(typeof next === 'function' ? next($overlayState.get()) : { ...$overlayState.get(), ...next })
 
+/** Full reset — used by session/turn teardown and tests. */
 export const resetOverlayState = () => $overlayState.set(buildOverlayState())
+
+/**
+ * Soft reset: drop FLOW-scoped overlays (approval / clarify / confirm / sudo
+ * / secret / pager) but PRESERVE user-toggled ones — agents dashboard, model
+ * picker, skills hub, session picker.  Those are opened deliberately and
+ * shouldn't vanish when a turn ends.  Called from turnController.idle() on
+ * every turn completion / interrupt; the old "reset everything" behaviour
+ * silently closed /agents the moment delegation finished.
+ */
+export const resetFlowOverlays = () =>
+  $overlayState.set({
+    ...buildOverlayState(),
+    agents: $overlayState.get().agents,
+    agentsInitialHistoryIndex: $overlayState.get().agentsInitialHistoryIndex,
+    modelPicker: $overlayState.get().modelPicker,
+    picker: $overlayState.get().picker,
+    skillsHub: $overlayState.get().skillsHub
+  })