feat(delegate): orchestrator role and configurable spawn depth (default flat)

Adds role='leaf'|'orchestrator' to delegate_task. With max_spawn_depth>=2,
an orchestrator child retains the 'delegation' toolset and can spawn its
own workers; leaf children cannot delegate further (identical to today).

Default posture is flat — max_spawn_depth=1 means a depth-0 parent's
children land at the depth-1 floor and orchestrator role silently
degrades to leaf. Users opt into nested delegation by raising
max_spawn_depth to 2 or 3 in config.yaml.

Also threads acp_command/acp_args through the main agent loop's delegate
dispatch (previously silently dropped in the schema) via a new
_dispatch_delegate_task helper, and adds a DelegateEvent enum with
legacy-string back-compat for gateway/ACP/CLI progress consumers.

Config (hermes_cli/config.py defaults):
  delegation.max_concurrent_children: 3   # floor-only, no upper cap
  delegation.max_spawn_depth: 1           # 1=flat (default), 2-3 unlock nested
  delegation.orchestrator_enabled: true   # global kill switch

Salvaged from @pefontana's PR #11215. Overrides vs. the original PR:
concurrency stays at 3 (PR bumped to 5 + cap 8 — we keep the floor only,
no hard ceiling); max_spawn_depth defaults to 1 (PR defaulted to 2 which
silently enabled one level of orchestration for every user).

Co-authored-by: pefontana <fontana.pedro93@gmail.com>

website/docs/user-guide/features/delegation.md

@@ -20,7 +20,7 @@ delegate_task(
 
 ## Parallel Batch
 
-Up to 3 concurrent subagents:
+Up to 3 concurrent subagents by default (configurable, no hard ceiling):
 
 ```python
 delegate_task(tasks=[
@@ -121,8 +121,8 @@ delegate_task(
 
 When you provide a `tasks` array, subagents run in **parallel** using a thread pool:
 
-- **Maximum concurrency:** 3 tasks (the `tasks` array is truncated to 3 if longer)
-- **Thread pool:** Uses `ThreadPoolExecutor` with `MAX_CONCURRENT_CHILDREN = 3` workers
+- **Maximum concurrency:** 5 tasks by default (configurable via `delegation.max_concurrent_children`, absolute cap of 8)
+- **Thread pool:** Uses `ThreadPoolExecutor` with the configured concurrency limit as max workers
 - **Progress display:** In CLI mode, a tree-view shows tool calls from each subagent in real-time with per-task completion lines. In gateway mode, progress is batched and relayed to the parent's progress callback
 - **Result ordering:** Results are sorted by task index to match input order regardless of completion order
 - **Interrupt propagation:** Interrupting the parent (e.g., sending a new message) interrupts all active children
@@ -154,8 +154,8 @@ The `toolsets` parameter controls what tools the subagent has access to. Choose
 | `["file"]` | Read-only analysis, code review without execution |
 | `["terminal"]` | System administration, process management |
 
-Certain toolsets are **always blocked** for subagents regardless of what you specify:
-- `delegation` — no recursive delegation (prevents infinite spawning)
+Certain toolsets are blocked for subagents regardless of what you specify:
+- `delegation` — blocked for leaf subagents (the default). Retained for `role="orchestrator"` children, bounded by `max_spawn_depth` — see [Depth Limit and Nested Orchestration](#depth-limit-and-nested-orchestration) below.
 - `clarify` — subagents cannot interact with the user
 - `memory` — no writes to shared persistent memory
 - `code_execution` — children should reason step-by-step
@@ -173,16 +173,32 @@ delegate_task(
 )
 ```
 
-## Depth Limit
+## Depth Limit and Nested Orchestration
 
-Delegation has a **depth limit of 2** — a parent (depth 0) can spawn children (depth 1), but children cannot delegate further. This prevents runaway recursive delegation chains.
+By default, delegation is **flat**: a parent (depth 0) spawns children (depth 1), and those children cannot delegate further. This prevents runaway recursive delegation.
+
+For multi-stage workflows (research → synthesis, or parallel orchestration over sub-problems), a parent can spawn **orchestrator** children that *can* delegate their own workers:
+
+```python
+delegate_task(
+    goal="Survey three code review approaches and recommend one",
+    role="orchestrator",  # Allows this child to spawn its own workers
+    context="...",
+)
+```
+
+- `role="leaf"` (default): child cannot delegate further — identical to the flat-delegation behavior.
+- `role="orchestrator"`: child retains the `delegation` toolset. Gated by `delegation.max_spawn_depth` (default **1** = flat, so `role="orchestrator"` is a no-op at defaults). Raise `max_spawn_depth` to 2 to allow orchestrator children to spawn leaf grandchildren; 3 for three levels (cap).
+- `delegation.orchestrator_enabled: false`: global kill switch that forces every child to `leaf` regardless of the `role` parameter.
+
+**Cost warning:** With `max_spawn_depth: 3` and `max_concurrent_children: 3`, the tree can reach 3×3×3 = 27 concurrent leaf agents. Each extra level multiplies spend — raise `max_spawn_depth` intentionally.
 
 ## Key Properties
 
 - Each subagent gets its **own terminal session** (separate from the parent)
-- **No nested delegation** — children cannot delegate further (no grandchildren)
-- Subagents **cannot** call: `delegate_task`, `clarify`, `memory`, `send_message`, `execute_code`
-- **Interrupt propagation** — interrupting the parent interrupts all active children
+- **Nested delegation is opt-in** — only `role="orchestrator"` children can delegate further, and only when `max_spawn_depth` is raised from its default of 1 (flat). Disable globally with `orchestrator_enabled: false`.
+- Leaf subagents **cannot** call: `delegate_task`, `clarify`, `memory`, `send_message`, `execute_code`. Orchestrator subagents retain `delegate_task` but still cannot use the other four.
+- **Interrupt propagation** — interrupting the parent interrupts all active children (including grandchildren under orchestrators)
 - Only the final summary enters the parent's context, keeping token usage efficient
 - Subagents inherit the parent's **API key, provider configuration, and credential pool** (enabling key rotation on rate limits)
 
@@ -193,7 +209,7 @@ Delegation has a **depth limit of 2** — a parent (depth 0) can spawn children
 | **Reasoning** | Full LLM reasoning loop | Just Python code execution |
 | **Context** | Fresh isolated conversation | No conversation, just script |
 | **Tool access** | All non-blocked tools with reasoning | 7 tools via RPC, no reasoning |
-| **Parallelism** | Up to 3 concurrent subagents | Single script |
+| **Parallelism** | 3 concurrent subagents by default (configurable) | Single script |
 | **Best for** | Complex tasks needing judgment | Mechanical multi-step pipelines |
 | **Token cost** | Higher (full LLM loop) | Lower (only stdout returned) |
 | **User interaction** | None (subagents can't clarify) | None |
@@ -206,6 +222,9 @@ Delegation has a **depth limit of 2** — a parent (depth 0) can spawn children
 # In ~/.hermes/config.yaml
 delegation:
   max_iterations: 50                        # Max turns per child (default: 50)
+  # max_concurrent_children: 3              # Parallel children per batch (default: 3)
+  # max_spawn_depth: 1                      # Tree depth (1-3, default 1 = flat). Raise to 2 to allow orchestrator children to spawn leaves; 3 for three levels.
+  # orchestrator_enabled: true              # Disable to force all children to leaf role.
   model: "google/gemini-3-flash-preview"             # Optional provider/model override
   provider: "openrouter"                             # Optional built-in provider