Create Cargo workspace at v2/ level with members: src-tauri,
bterminal-core, bterminal-relay. Extract PtyManager and SidecarManager
into shared bterminal-core crate with EventSink trait for abstracting
event emission. TauriEventSink wraps AppHandle. src-tauri pty.rs and
sidecar.rs become thin re-exports. Move Cargo.lock to workspace root.
Add v2/target/ to .gitignore.
Refactor SidecarManager to use SidecarCommand struct abstracting
runtime choice. resolve_sidecar_command() prefers Deno (runs TS
directly, no build step), falls back to Node.js if deno not in PATH.
Both runners bundled in tauri.conf.json resources. Set auto-update
signing pubkey in updater config.
Phase 3 polish: dispatcher listens for sidecar-exited events and marks
running sessions as error. AgentPane shows "Restart Sidecar" button on
error. Auto-scroll disables when user scrolls >50px from bottom with
"Scroll to bottom" button. Added agent_restart Tauri command.
