docs: document delegation width + depth knobs (#13745)

Fills the three gaps left by the orchestrator/width-depth salvage:

- configuration.md §Delegation: max_concurrent_children, max_spawn_depth,
  orchestrator_enabled are now in the canonical config.yaml reference
  with a paragraph covering defaults, clamping, role-degradation, and
  the 3x3x3=27-leaf cost scaling.
- environment-variables.md: adds DELEGATION_MAX_CONCURRENT_CHILDREN to
  the Agent Behavior table.
- features/delegation.md: corrects stale 'default 5, cap 8' wording
  (that was from the original PR; the salvage landed on default 3 with
  no ceiling and a tool error on excess instead of truncation).

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

@@ -121,7 +121,7 @@ delegate_task(
 
 When you provide a `tasks` array, subagents run in **parallel** using a thread pool:
 
-- **Maximum concurrency:** 5 tasks by default (configurable via `delegation.max_concurrent_children`, absolute cap of 8)
+- **Maximum concurrency:** 3 tasks by default (configurable via `delegation.max_concurrent_children` or the `DELEGATION_MAX_CONCURRENT_CHILDREN` env var; floor of 1, no hard ceiling). Batches larger than the limit return a tool error rather than being silently truncated.
 - **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