Feature: Team delegation from todos — assign todo to sub-agent session

[Delqhi]

Problem

ECC has ecc delegate --from-session <id> --task "..." which creates a child session with a TaskHandoff message. Our orchestrator can spawn sub-agents but todos can't be delegated to parallel sessions. There's no sin-code todo delegate command.

What to do

1. sin-code todo delegate <id> --to <agent-type>:
   • Creates a new session (or worktree) for the todo
   • Sends TaskHandoff message (requires typed messaging issue)
   • Sets todo.Assignee = "session:<id>"
   • Sets todo.Status = "in_progress"
2. sin-code todo delegate <id> --to <agent-type> --worktree:
   • Creates isolated git worktree for the delegate
3. sin-code team status <id> — show delegation tree for a todo
4. On delegate session completion → auto-complete todo
5. TUI: delegated todos show session tree icon (🌳)

Acceptance criteria

• sin-code todo delegate creates a session
• --worktree creates isolated worktree
• team status shows delegation tree
• Auto-complete on session completion
• TUI shows delegation status

[Delqhi]

Implemented and committed to main.