feat: dual-mode sessions (TUI + stream-json) with prompt API

[paolino]

Context

Agent-daemon currently runs claude CLI in TUI mode inside tmux sessions, exposed via WebSocket for terminal clients (kanbanned TUI and browser). To support non-terminal clients (Telegram), we need a second mode where the claude process runs with --output-format stream-json --input-format stream-json, exposing a prompt-level API.

Terminology

• Session = the tmux session, tied to a worktree. Lives until explicitly killed (issue work is done). DELETE /sessions/<id> kills the session.
• Mode = how the claude process runs inside that tmux session: terminal (TUI) or structured (stream-json). Switching mode restarts the claude process, not the tmux session.
• Resume = --resume {session_id} carries conversation history across claude process restarts within the same tmux session. Same cwd, same worktree, same session files — just different process flags.

Requirements

Dual-mode claude process

• The claude process inside a tmux session can run in either terminal mode (TUI, existing) or structured mode (stream-json)
• Mode is part of session state, visible in GET /sessions response
• Switching mode: kill the current claude process, respawn with --resume {session_id} and the appropriate flags, inside the same tmux session
• The tmux session and worktree are unaffected by mode switches
• Mutual exclusion: only one claude process per session at a time

Prompt API (structured mode only)

• POST /sessions/<id>/prompt — send a text prompt, stream back JSON response
• Response streaming via SSE or WebSocket (TBD)
• Permission callbacks forwarded to the caller (for Telegram approval buttons)

Mode switching

• POST /sessions/<id>/mode — switch between terminal and structured
• Uses --resume to carry conversation history across mode switches
• Same tmux session, same cwd — resume should work naturally since session files stay in place

CLI flags for structured mode

Based on how claude-agent-sdk works internally:

claude --output-format stream-json \
       --input-format stream-json \
       --permission-prompt-tool stdio \
       --resume {session_id}

The daemon reads/writes JSON on the process stdin/stdout instead of piping terminal I/O.

Session lifecycle (unchanged)

• POST /sessions — creates tmux session + worktree + launches claude (TUI by default)
• POST /sessions/<id>/mode — restarts claude process with different flags (NEW)
• DELETE /sessions/<id> — kills tmux session entirely, optionally cleans up worktree (existing, unchanged)

Non-goals

• Replacing terminal mode — terminal clients (kanbanned TUI, browser) continue using the existing WebSocket terminal API
• Python SDK dependency — the daemon speaks the JSON protocol directly