feat(tui): add /subagents command to view subagent tree for current session

[adam2am]

Summary

Adds a /subagents command that displays a hierarchical tree of all subagents spawned during the current session, making it easy to track parallel agent execution.

• Tree view: Shows parent → child relationships of all spawned subagents from root
• Current session highlight: Marks your active position in the tree
• Configurable keybind: session_subagents (default: none)

Changes

File	Description
src/cli/cmd/tui/component/dialog-subagent-list.tsx	Dialog component for subagent tree
src/cli/cmd/tui/util/subagent-tree.ts	Tree building utilities
src/cli/cmd/tui/app.tsx	Register keybind handler
src/config/config.ts	Add session_subagents keybind
test/cli/subagent-tree.test.ts	35 unit tests

Testing

bun test test/cli/subagent-tree.test.ts — all 35 tests pass