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/theme.ts

@@ -94,7 +94,12 @@ export const DARK_THEME: Theme = {
     amber: '#FFBF00',
     bronze: '#CD7F32',
     cornsilk: '#FFF8DC',
-    dim: '#B8860B',
+    // Bumped from the old `#B8860B` darkgoldenrod (~53% luminance) which
+    // read as barely-visible on dark terminals for long body text.  The
+    // new value sits ~60% luminance — readable without losing the "muted /
+    // secondary" semantic.  Field labels still use `label` (65%) which
+    // stays brighter so hierarchy holds.
+    dim: '#CC9B1F',
     completionBg: '#FFFFFF',
     completionCurrentBg: mix('#FFFFFF', '#FFBF00', 0.25),
 
@@ -104,8 +109,11 @@ export const DARK_THEME: Theme = {
     warn: '#ffa726',
 
     prompt: '#FFF8DC',
-    sessionLabel: '#B8860B',
-    sessionBorder: '#B8860B',
+    // sessionLabel/sessionBorder intentionally track the `dim` value — they
+    // are "same role, same colour" by design.  fromSkin's banner_dim fallback
+    // relies on this pairing (#11300).
+    sessionLabel: '#CC9B1F',
+    sessionBorder: '#CC9B1F',
 
     statusBg: '#1a1a2e',
     statusFg: '#C0C0C0',