feat: Nested agent composition with parent-child execution graphs

[aecs4u]

Summary

Enable agents to compose other agents as sub-tasks, forming a parent-child execution tree with result aggregation and error propagation.

Prior Art

• AutoGPT implements nested agent composition as a core feature — agent graphs can contain "Agent Executor" blocks that run other agents as sub-steps. Parent agents receive structured results from child agents and can branch/retry based on outcomes. Supports up to 3 levels of nesting with shared context propagation.
• Moltis has a similar concept via its multi-agent orchestration layer.
• RustyClaw already has sessions_spawn (feat: Sub-agent / spawn_agent tool for parallel task delegation #66) for basic sub-agent delegation.

Motivation

The current sub-agent system (#66) supports spawning independent sessions, but lacks structured composition — a parent agent can't easily define "run agent A, then if successful run agent B with A's output, otherwise run agent C". Nested composition enables complex workflows like: research agent → analysis agent → report-writing agent, with the parent orchestrating the pipeline.

Proposed Design

1. Extend sessions_spawn with composition semantics:

   struct AgentComposition {
       steps: Vec<CompositionStep>,
       error_strategy: ErrorStrategy, // Retry, Fallback, Abort
       max_depth: u8,                 // Default: 3
   }
   
   enum CompositionStep {
       Sequential(Vec<AgentTask>),
       Parallel(Vec<AgentTask>),
       Conditional { condition: String, if_true: AgentTask, if_false: AgentTask },
   }

2. Parent agent receives structured results from each step
3. Resource limits inherited and subdivided (budget, time, sandbox scope)
4. Execution tree visible in TUI and gateway status

Acceptance Criteria

• Sequential and parallel agent composition
• Conditional branching based on sub-agent results
• Max nesting depth enforced (default: 3)
• Resource limits (budget, time) subdivided across children
• Execution tree visualization in TUI

Related Issues

• feat: Sub-agent / spawn_agent tool for parallel task delegation #66 (Sub-agent / spawn_agent tool — this extends it)
• feat: Multi-agent routing and isolation #61 (Multi-agent routing and isolation)
• feat: Agent event streaming for real-time progress tracking #87 (Agent event streaming — tree status events)