feat: Session Phase 2 — interactive agent chat + split pane

[jonwiggins]

Summary

Add an interactive Claude Code chat pane alongside the existing web terminal in sessions. This is Phase 2 of the sessions feature (#80) — Phase 1 (terminal + PR tracking) shipped in #93.

Motivation

Sessions currently give users a terminal into a repo pod, but no way to interact with an AI agent within that same workspace. The goal is a side-by-side experience: agent chat on the left, terminal on the right, both operating on the same worktree.

Features

Split pane layout

• Resizable split — agent chat (left) + terminal (right)
• Either pane collapsible to full-width
• Layout preference persisted (localStorage or session record)

Interactive agent chat

• Long-running claude process exec'd in the pod (not one-shot per message) — stdin/stdout piped through WebSocket (WS /ws/sessions/:id/chat)
• Streaming responses rendered in a chat pane with markdown
• Tool use visibility — show files being read/edited and commands being run (reuse existing log viewer / agent event parser)
• Interrupt button — cancel the current response mid-stream
• Session history persists in scrollback even when navigating away and back

Terminal ↔ Agent coordination

• "Send to agent" action — select terminal output and send it as context to the chat (e.g. paste an error and say "fix this")
• Shared worktree means git status in the terminal immediately reflects agent edits and vice versa

Cost and tracking

• Live cost counter displayed in the session header (token spend so far)
• Per-message cost breakdown in the chat UI
• Model selector — pick which model the agent uses (default from repo claudeModel setting)

PR lifecycle (extending Phase 1)

• Trigger review agent on any session PR directly from the chat pane
• CI status / review status badges inline when a PR is detected

API

• WS /ws/sessions/:id/chat — agent chat WebSocket (stdin/stdout + structured events)
• Extend GET /api/sessions/:id to include cost breakdown and model info
• Extend POST /api/sessions/:id/end to kill the claude process if running

Non-goals

• Multiple terminal tabs per session
• Auto-fix CI failures (user-driven only)
• Multi-agent (one claude process per session)
• Control plane agent ("chat with Optio" outside a repo context)

Follows

• feat: Interactive sessions — terminal + agent chat for repo pods #80 (original sessions spec)
• feat: add interactive sessions with terminal and PR tracking #93 (Phase 1 implementation)

[jonwiggins]

Optio is working on this issue.

Task ID: 3a9380fd-160f-4234-b36a-e0cf1748ef42
Agent: claude-code

[jonwiggins]

🤖 Optio is working on this issue.

Task ID: c646b7df-6366-46fe-9269-53bb3cdc44b3
Agent: claude-code

[jonwiggins]

✅ Optio completed this issue. Changes merged in #109.