feat(cli): add interactive multi-turn REPL mode ('terminal-agent chat')

[Kaap10]

Problem Statement

Currently, terminal-agent run <TASK> is a single-shot batch command that exits after completing or failing a task. Developers cannot converse with the agent, ask follow-up questions, request iterative refactors, or inspect intermediate plans in real-time.

Proposed Architecture & Solution

1. Interactive REPL Command (src/terminal_agent/cli/commands/chat.py):
   • Create a dedicated interactive shell command terminal-agent chat (or bare terminal-agent in interactive terminals).
   • Maintain active SessionState and message history across turns.
2. In-Session Slash Commands:
   • /diff: Display current git working tree diff.
   • /verify: Manually trigger the independent test verification suite.
   • /checkpoint [name]: Create a snapshot of current progress.
   • /rollback [id]: Restore files to a previous checkpoint.
   • /status: Show step count, failures, and modified files.
   • /exit or /quit: Cleanly finalize and save session state.
3. Rich Terminal Interface:
   • Utilize prompt_toolkit or rich.prompt with command history, syntax highlighting, and auto-completion.

Files to Modify / Create

• src/terminal_agent/cli/commands/chat.py (New)
• src/terminal_agent/cli/main.py (Register chat command and interactive routing)
• src/terminal_agent/agent/loop.py (Add step-by-step turn execution support)
• tests/unit/test_chat_command.py (New)

Acceptance Criteria

• terminal-agent chat launches an interactive multi-turn session.
• Slash commands (/diff, /verify, /checkpoint, /status) execute correctly without interrupting session state.
• Session is saved to .terminal_agent/sessions/<id>.json on exit and can be resumed via terminal-agent resume <id>.
• Automated CLI invocation tests added.