Add read-only subagent command UI

[tasercake]

Summary

• add /subagents and Ctrl+Shift+S entry points to one read-only TUI
• show current-session subagents with reconciled status, elapsed time, and terminal-safe prompt previews
• support Up/Down selection, Enter details, and Escape close without steering or mutation controls
• preserve read-only inspection tools merged in Add read-only subagent inspection tools #48 and document TUI usage
• add focused command, shortcut, navigation, sanitization, status, details, and headless-guard tests

Validation

• node --experimental-strip-types --test test/unit/subagents-ui.test.ts (5 passed)
• npm run test:unit (72 passed, 1 skipped integration gate)
• rebased onto origin/main at merge commit 3ae3ca0 (Add read-only subagent inspection tools #48)