feat(agent): orchestrate multiple independent tasks via separate sub-agents #4

Problem

Users want to give zap multiple independent tasks in one request and have each handled separately instead of serially in a single agent loop.

Goal

Add a batch orchestration flow where one parent run can fan out N independent tasks to N sub-agents.

Expected UX

• zap orchestrate tasks.yaml
• zap orchestrate --task \"fix auth tests\" --task \"update docs\" --task \"review diff\"
• agent-side workflow that maps one task to one sub-agent automatically

Requirements

• one sub-agent per task
• parallel execution for independent tasks
• explicit per-task context and optional file scope
• parent summary when all tasks complete
• safe handling of overlapping write scopes

Likely implementation areas

• src/agent_core.rs
• sub-agent spawning/tool orchestration
• CLI/TUI entrypoints

Acceptance criteria

• multiple tasks can be submitted in one run
• each task executes independently
• results are collected and summarized
• overlapping file writes are detected or blocked
  EOF