Delegate to subagents or manage agent definitions. See the `agent` parameter for the catalog of available specialists and their capabilities.

EXECUTION (use exactly ONE mode):
• SINGLE: { agent, task } - one task
• CHAIN: { chain: [...] } - sequential pipeline with optional parallel fan-out
• PARALLEL: { tasks: [{agent,task,count?}, ...], concurrency?: number, worktree?: true } - concurrent execution (worktree: isolate each task in a git worktree)
• Optional context: { context: "fresh" | "fork" } (default: "fresh")

CHAIN TEMPLATE VARIABLES (use in task strings):
• {task} - The original task/request from the user
• {previous} - Text response from the previous step (empty for first step)
• {chain_dir} - Shared directory for chain files (e.g., <tmpdir>/pi-subagents-<scope>/chain-runs/abc123/)

MANAGEMENT (use action field, omit agent/task/chain/tasks):
• { action: "list" } - discover agents/chains
• { action: "get", agent: "name" } - full detail
• { action: "create", config: { name, systemPrompt, systemPromptMode, inheritProjectContext, inheritSkills, ... } }
• { action: "update", agent: "name", config: { ... } } - merge
• { action: "delete", agent: "name" }
• Use chainName for chain operations

CONTROL:
• { action: "status", id: "..." } - inspect an async/background run by id or prefix
• { action: "interrupt", id?: "..." } - soft-interrupt the current child turn and leave the run paused
