feat: shared iteration budget across parent + subagents

Subagent tool calls now count toward the same session-wide iteration
limit as the parent agent. Previously, each subagent had its own
independent counter, so a parent with max_iterations=60 could spawn
3 subagents each doing 50 calls = 150 total tool calls unmetered.

Changes:
- IterationBudget: thread-safe shared counter (run_agent.py)
  - consume(): try to use one iteration, returns False if exhausted
  - refund(): give back one iteration (for execute_code turns)
  - Thread-safe via Lock (subagents run in ThreadPoolExecutor)
- Parent creates the budget, children inherit it via delegate_tool.py
- execute_code turns are refunded (don't count against budget)
- Default raised from 60 → 90 to account for shared consumption
- Per-child cap (50) still applies as a safety valve

The per-child max_iterations (default 50) remains as a per-child
ceiling, but the shared budget is the hard session-wide limit.
A child stops at whichever comes first.

gateway/run.py

@@ -2097,7 +2097,7 @@ class GatewayRunner:
             os.environ["HERMES_SESSION_KEY"] = session_key or ""
 
             # Read from env var or use default (same as CLI)
-            max_iterations = int(os.getenv("HERMES_MAX_ITERATIONS", "60"))
+            max_iterations = int(os.getenv("HERMES_MAX_ITERATIONS", "90"))
             
             # Map platform enum to the platform hint key the agent understands.
             # Platform.LOCAL ("local") maps to "cli"; others pass through as-is.