fix(delegation): isolate subagent iteration budgets by default (fixes #2873)

[Mibayy]

Problem

Subagents consistently exit with exit_reason: max_iterations before completing their task, even when max_iterations is set explicitly. This is especially bad in batch mode with 3 parallel subagents.

Root cause: _build_child_agent() passes the parent's IterationBudget object directly to each child. That budget is already partially consumed by the parent's own turns. In parallel batch mode, all 3 children share what's left and race to exhaust it.

# Parent used 30 of 90 turns → 60 remaining, shared by all children
# 3 subagents each requested max_iterations=50

Subagent 1: gets ~20 turns → exits mid-task  ✗
Subagent 2: gets ~20 turns → exits mid-task  ✗
Subagent 3: gets ~20 turns → exits mid-task  ✗

Fix

Introduce a budget_mode config option under delegation: in config.yaml.

isolated (new default)

Each child gets a fresh IterationBudget(max_iterations) starting at 0. max_iterations=50 means exactly 50 turns, independent of parent consumption or parallel siblings.

# Same scenario with budget_mode: isolated (default)
# Parent used 30 of 90 turns — irrelevant to children

Subagent 1:  0/50 fresh budget → uses all 50 turns  ✓
Subagent 2:  0/50 fresh budget → uses all 50 turns  ✓
Subagent 3:  0/50 fresh budget → uses all 50 turns  ✓

shared (legacy)

Preserves the old behaviour for users who explicitly want a session-wide cap across parent + all children combined. Not recommended but kept for backwards compatibility.

Changes

• tools/delegate_tool.py — read budget_mode from config in _build_child_agent(); pass iteration_budget=None (isolated) or the parent's budget object (shared). Added module-level docstring explaining both modes with concrete examples.
• website/docs/user-guide/features/delegation.md — new "Iteration Budget Modes" section with visual examples (bar charts), updated Configuration table.

Config

delegation:
  max_iterations: 50      # per-subagent cap
  budget_mode: isolated   # "isolated" (default) or "shared" (legacy)

Closes #2873

[teknium1]

This was already fixed on main in commit 68ab37e (PR #3004), which sets iteration_budget=None so each subagent gets a fresh independent budget. The core bug is resolved. The additional budget_mode: shared config toggle for the old broken behavior isn't something we want to maintain. Thanks for the report and detailed writeup though — the problem description was spot-on.