Interactive agent mode (epic)

Add agent -i / agent --interactive mode for multi-turn chat-style conversations.

Design

See _/tmp/interactive-agent-design.md

Architecture

Option A: Wrapper around existing runAgent, rebuild prompt with conversation history each turn.

Key Decisions

• Tool output: truncated inline (10 lines), expand with /expand
• Memory: use MEMORY_USER_ID env var (same as non-interactive)
• Input: haskeline for readline support
• Flags: both -i and --interactive

Non-Goals (v1)

• Session persistence (use --checkpoint-dir)
• Streaming responses
• Brick TUI
• Branching/forking