docs: record omp subagent fan-out caps and per-endpoint in-flight ceilings

Codifies the limits set in agent/config.yml 2026-10-19: task concurrency 2,
no nested subagent spawns (maxRecursionDepth 1 - and why 0 must never be
set), 30-min runtime / 120-request budgets, isolation opt-in enabled,
completion probes off, 600s tool timeout ceiling, and per-provider
in-flight caps (archon 3, dragoon 2, zealot 3) keyed by models.yml provider
ids with their per-machine scope caveat. Small docs-only change; no issue
opened per trunk-based small-fix exemption.

omp-docs/pi/index.md

@@ -28,7 +28,8 @@
 
 ## Subagent Architecture
 
-- [Pi Subagents](/pi/packages/pi-subagents.md) — Core subagent delegation system. Chains, parallel execution, TUI clarification.
+- [OMP Subagent Limits & Fan-out Policy](/pi/subagent-limits.md) — Live caps: concurrency 2, no nested spawns (and the `maxRecursionDepth: 0` trap), per-endpoint in-flight ceilings, isolation policy.
+- [Pi Subagents](/pi/packages/pi-subagents.md) — (historical) pi-era delegation system. Chains, parallel execution, TUI clarification.
 
 ## Architecture