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/spawnHistoryStore.ts

@@ -0,0 +1,139 @@
+import { atom } from 'nanostores'
+
+import type { SpawnTreeLoadResponse } from '../gatewayTypes.js'
+import type { SubagentProgress } from '../types.js'
+
+export interface SpawnSnapshot {
+  finishedAt: number
+  fromDisk?: boolean
+  id: string
+  label: string
+  path?: string
+  sessionId: null | string
+  startedAt: number
+  subagents: SubagentProgress[]
+}
+
+export interface SpawnDiffPair {
+  baseline: SpawnSnapshot
+  candidate: SpawnSnapshot
+}
+
+const HISTORY_LIMIT = 10
+
+export const $spawnHistory = atom<SpawnSnapshot[]>([])
+export const $spawnDiff = atom<null | SpawnDiffPair>(null)
+
+export const getSpawnHistory = () => $spawnHistory.get()
+export const getSpawnDiff = () => $spawnDiff.get()
+
+export const clearSpawnHistory = () => $spawnHistory.set([])
+export const clearDiffPair = () => $spawnDiff.set(null)
+export const setDiffPair = (pair: SpawnDiffPair) => $spawnDiff.set(pair)
+
+/**
+ * Commit a finished turn's spawn tree to history.  Keeps the last 10
+ * non-empty snapshots — empty turns (no subagents) are dropped.
+ *
+ * Why in-memory?  The primary investigation loop is "I just ran a fan-out,
+ * it misbehaved, let me look at what happened" — same-session debugging.
+ * Disk persistence across process restarts is a natural extension but
+ * adds RPC surface for a less-common path.
+ */
+export const pushSnapshot = (
+  subagents: readonly SubagentProgress[],
+  meta: { sessionId?: null | string; startedAt?: null | number }
+) => {
+  if (!subagents.length) {
+    return
+  }
+
+  const now = Date.now()
+  const started = meta.startedAt ?? Math.min(...subagents.map(s => s.startedAt ?? now))
+
+  const snap: SpawnSnapshot = {
+    finishedAt: now,
+    id: `snap-${now.toString(36)}`,
+    label: summarizeLabel(subagents),
+    sessionId: meta.sessionId ?? null,
+    startedAt: Number.isFinite(started) ? started : now,
+    subagents: subagents.map(item => ({ ...item }))
+  }
+
+  const next = [snap, ...$spawnHistory.get()].slice(0, HISTORY_LIMIT)
+  $spawnHistory.set(next)
+}
+
+function summarizeLabel(subagents: readonly SubagentProgress[]): string {
+  const top = subagents
+    .filter(s => s.parentId == null || subagents.every(o => o.id !== s.parentId))
+    .slice(0, 2)
+    .map(s => s.goal || 'subagent')
+    .join(' · ')
+
+  return top || `${subagents.length} agent${subagents.length === 1 ? '' : 's'}`
+}
+
+/**
+ * Push a disk-loaded snapshot onto the front of the history stack so the
+ * overlay can pick it up at index 1 via /replay load.  Normalises the
+ * server payload (arbitrary list) into the same SubagentProgress shape
+ * used for live data — defensive against cross-version reads.
+ */
+export const pushDiskSnapshot = (r: SpawnTreeLoadResponse, path: string) => {
+  const raw = Array.isArray(r.subagents) ? r.subagents : []
+  const normalised = raw.map(normaliseSubagent)
+
+  if (!normalised.length) {
+    return
+  }
+
+  const snap: SpawnSnapshot = {
+    finishedAt: (r.finished_at ?? Date.now() / 1000) * 1000,
+    fromDisk: true,
+    id: `disk-${path}`,
+    label: r.label || `${normalised.length} subagents`,
+    path,
+    sessionId: r.session_id ?? null,
+    startedAt: (r.started_at ?? r.finished_at ?? Date.now() / 1000) * 1000,
+    subagents: normalised
+  }
+
+  const next = [snap, ...$spawnHistory.get()].slice(0, HISTORY_LIMIT)
+  $spawnHistory.set(next)
+}
+
+function normaliseSubagent(raw: unknown): SubagentProgress {
+  const o = raw as Record<string, unknown>
+  const s = (v: unknown) => (typeof v === 'string' ? v : undefined)
+  const n = (v: unknown) => (typeof v === 'number' ? v : undefined)
+  const arr = <T>(v: unknown): T[] | undefined => (Array.isArray(v) ? (v as T[]) : undefined)
+
+  return {
+    apiCalls: n(o.apiCalls),
+    costUsd: n(o.costUsd),
+    depth: typeof o.depth === 'number' ? o.depth : 0,
+    durationSeconds: n(o.durationSeconds),
+    filesRead: arr<string>(o.filesRead),
+    filesWritten: arr<string>(o.filesWritten),
+    goal: s(o.goal) ?? 'subagent',
+    id: s(o.id) ?? `sa-${Math.random().toString(36).slice(2, 8)}`,
+    index: typeof o.index === 'number' ? o.index : 0,
+    inputTokens: n(o.inputTokens),
+    iteration: n(o.iteration),
+    model: s(o.model),
+    notes: (arr<string>(o.notes) ?? []).filter(x => typeof x === 'string'),
+    outputTail: arr(o.outputTail) as SubagentProgress['outputTail'],
+    outputTokens: n(o.outputTokens),
+    parentId: s(o.parentId) ?? null,
+    reasoningTokens: n(o.reasoningTokens),
+    startedAt: n(o.startedAt),
+    status: (s(o.status) as SubagentProgress['status']) ?? 'completed',
+    summary: s(o.summary),
+    taskCount: typeof o.taskCount === 'number' ? o.taskCount : 1,
+    thinking: (arr<string>(o.thinking) ?? []).filter(x => typeof x === 'string'),
+    toolCount: typeof o.toolCount === 'number' ? o.toolCount : 0,
+    tools: (arr<string>(o.tools) ?? []).filter(x => typeof x === 'string'),
+    toolsets: arr<string>(o.toolsets)
+  }
+}