feat(mcp): delegate presets for start_session and fan_out #3391

Problem

Every delegation restates harness, model, effort, account, permissions and the same boilerplate instructions (read AGENTS.md, timeouts, no attribution). Orchestrator prompts get long and drift.

Proposal

User-defined presets (Settings, stored in config): name, harness, model, effort, account, access, worktree default, and an instructions block prepended to the prompt. start_session/fan_out take preset; explicit fields override it. The UI can start a delegate from a preset too.

Acceptance criteria

• A preset can be created in Settings and used by name from the MCP tools.
• Explicit arguments override preset values; unknown preset is a clear error.

Reference: cloudroom's tasks plugin delegate presets.