Extract TUI layer into standalone `clemitui` crate

[evansenter]

Summary

Extract the terminal UI layer (formatting, event handling, text buffering, logging, REPL) into a standalone clemitui crate published on crates.io. This enables any ACP-compatible agent to use the TUI, similar to how Toad works with Claude.

After extraction, clemini becomes an ACP-only agent with no built-in terminal UI.

Motivation

• Reusability: The TUI can work with any ACP agent, not just clemini
• Separation of concerns: Agent logic vs presentation layer
• Ecosystem: Other LLM CLI projects can use clemitui

Approach: Minimal Extraction

Move code as-is with minimal changes. Only refactor coupling points.

What moves to clemitui

Module	Description
format.rs	TextBuffer, markdown rendering, format_tool_* functions
events.rs	EventHandler trait, TerminalEventHandler
logging.rs	OutputSink trait, log_event functions
REPL loop	From main.rs, spawns ACP subprocess

Key refactor

Change format functions to take primitives instead of genai-rs types:

// Before (coupled to genai-rs)
pub fn format_tool_result(result: &FunctionExecutionResult) -> String

// After (standalone)
pub fn format_tool_result(name: &str, duration_ms: u64, tokens: Option<u64>) -> String

Target architecture

clemitui crate (new):
├── format.rs         # Pure formatting (no genai-rs types)
├── events.rs         # EventHandler trait
├── logging.rs        # OutputSink trait
├── text_buffer.rs    # TextBuffer for streaming markdown
└── repl.rs           # REPL loop (spawns ACP subprocess)

clemini (simplified):
├── agent.rs          # Core interaction logic
├── acp.rs            # ACP server (becomes default mode)
├── tools/            # Tool implementations
└── main.rs           # Starts ACP server

Dependencies

• clemitui will depend on agent-client-protocol for ACP types
• clemitui will NOT depend on genai-rs

Design decisions

• ACP coupling is intentional: Since clemitui targets ACP agents, depending on ACP types in the public API is correct (ensures protocol compatibility)
• Primitives in format functions: Makes formatting testable and documentation-friendly without requiring protocol knowledge
• YAGNI on abstractions: Deferred introducing UiMode trait or AcpClient trait until there's concrete need

Tasks

Phase 1: Core extraction (PR #101) ✅

• Create clemitui crate structure
• Refactor format.rs to take primitives instead of genai-rs types
• Move TextBuffer to clemitui
• Move OutputSink trait and logging to clemitui
• Update documentation

Phase 2: EventHandler extraction (future)

• Move EventHandler trait to clemitui
• Move TerminalEventHandler to clemitui

Phase 3: REPL extraction (future)

• Extract REPL loop with ACP subprocess spawning
• Update clemini to be ACP-only (remove terminal UI code)

Phase 4: Publishing (future)

• Add clemitui to crates.io

Future considerations

If needed later, these can be added without breaking changes:

• UiMode trait for different rendering strategies (TUI library, headless)
• AcpClient trait for testing without real subprocess
• SessionEvent abstraction to hide ACP types from users