[Feature]: Chat-based UI

## Problem or use case

Orca's primary interaction model today is worktree- and terminal-centric. For many agent workflows—especially quick prompts, follow-ups, and mobile check-ins—a dedicated chat surface would be a more natural primary UI than navigating terminals and file trees.

A chat-based UI would let users converse with agents in a familiar message thread, see streaming responses in one place, and send follow-ups without re-focusing a terminal pane.

## Proposed solution

Add a first-class chat-based UI in Orca as an alternative (or complementary) interaction surface for agent work:

- Message-thread view with streaming agent output
- Follow-up prompts in the same conversation context
- Clear linkage between a chat thread and its backing worktree/terminal/session
- Usable from desktop and mobile for lightweight agent steering

This would improve discoverability for users who think in terms of "talk to the agent" rather than "manage terminals and worktrees."

## Alternatives or additional context

**Reference OSS:** [Paseo](https://github.com/getpaseo/paseo) implements a chat-first interface for coding agents across Claude Code, Codex, Copilot, OpenCode, and Pi—with desktop, mobile, web, and CLI clients connecting to a local daemon. Worth studying for UX patterns around multi-provider agent chat, cross-device continuity, and voice/hands-free interaction.

- https://github.com/getpaseo/paseo
- https://paseo.sh/docs