feat: interactive TUI for agent execution and session management

[Cloud370]

Motivation

The current kuku REPL (bare kuku without arguments) is minimal: no session management, only y/n permissions, no subagent visibility, no completion. Users need an interactive interface to fully leverage the SDK's capabilities.

Proposal

Build a TUI with ratatui + crossterm, replacing the bare REPL when kuku is invoked without arguments:

• Session management: auto-continue latest session, session switching panel
• Structured streaming output: collapsible thinking blocks, turn markers, highlighted tool calls/results
• Permission interaction: all four PermissionChoice options (Once/Session/Project/Deny)
• Subexec views: Tab into agent/tool sub-execution for real-time observation
• Tab completion: built-in commands + skill names
• Extensible architecture: model switching, theme customization (future)

Non-TUI modes (kuku run "prompt", piped, --json, --stream-json) are unaffected.

Alternatives considered

• crossterm-only line mode (original plan): simpler but can't do view switching for subagents, no fixed input area, no structured output navigation. Rejected because subagent observation is a core requirement.
• reedline standalone: excellent line editor but no output rendering capability at all.
• iocraft: promising declarative API but small ecosystem (1.3k stars), tokio compatibility uncertain.
• cursive: widget model oriented toward forms/menus, not REPL-style interfaces.

Affected scope

SDK + CLI