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/gatewayTypes.ts

@@ -280,15 +280,85 @@ export interface ReloadMcpResponse {
 // ── Subagent events ──────────────────────────────────────────────────
 
 export interface SubagentEventPayload {
+  api_calls?: number
+  cost_usd?: number
+  depth?: number
   duration_seconds?: number
+  files_read?: string[]
+  files_written?: string[]
   goal: string
-  status?: 'completed' | 'failed' | 'interrupted' | 'running'
+  input_tokens?: number
+  iteration?: number
+  model?: string
+  output_tail?: { is_error?: boolean; preview?: string; tool?: string }[]
+  output_tokens?: number
+  parent_id?: null | string
+  reasoning_tokens?: number
+  status?: 'completed' | 'failed' | 'interrupted' | 'queued' | 'running'
+  subagent_id?: string
   summary?: string
   task_count?: number
   task_index: number
   text?: string
+  tool_count?: number
   tool_name?: string
   tool_preview?: string
+  toolsets?: string[]
+}
+
+// ── Delegation control RPCs ──────────────────────────────────────────
+
+export interface DelegationStatusResponse {
+  active?: {
+    depth?: number
+    goal?: string
+    model?: null | string
+    parent_id?: null | string
+    started_at?: number
+    status?: string
+    subagent_id?: string
+    tool_count?: number
+  }[]
+  max_concurrent_children?: number
+  max_spawn_depth?: number
+  paused?: boolean
+}
+
+export interface DelegationPauseResponse {
+  paused?: boolean
+}
+
+export interface SubagentInterruptResponse {
+  found?: boolean
+  subagent_id?: string
+}
+
+// ── Spawn-tree snapshots ─────────────────────────────────────────────
+
+export interface SpawnTreeListEntry {
+  count: number
+  finished_at?: number
+  label?: string
+  path: string
+  session_id?: string
+  started_at?: number | null
+}
+
+export interface SpawnTreeListResponse {
+  entries?: SpawnTreeListEntry[]
+}
+
+export interface SpawnTreeLoadResponse {
+  finished_at?: number
+  label?: string
+  session_id?: string
+  started_at?: null | number
+  subagents?: unknown[]
+}
+
+export interface SpawnTreeSaveResponse {
+  path?: string
+  session_id?: string
 }
 
 export type GatewayEvent =
@@ -320,6 +390,7 @@ export type GatewayEvent =
   | { payload: { env_var: string; prompt: string; request_id: string }; session_id?: string; type: 'secret.request' }
   | { payload: { task_id: string; text: string }; session_id?: string; type: 'background.complete' }
   | { payload: { text: string }; session_id?: string; type: 'btw.complete' }
+  | { payload: SubagentEventPayload; session_id?: string; type: 'subagent.spawn_requested' }
   | { payload: SubagentEventPayload; session_id?: string; type: 'subagent.start' }
   | { payload: SubagentEventPayload; session_id?: string; type: 'subagent.thinking' }
   | { payload: SubagentEventPayload; session_id?: string; type: 'subagent.tool' }