native: composer & task creation from the apps

[itomek]

Part of #125. Blocked by the macOS shell.

Problem

Watching isn't enough — the apps must create tasks and steer running ones.

Outcome

Composer + task creation: message input sending task:input (text + CR), a new-task sheet (workspace picker + prompt), and an interrupt control (task:interrupt).

Acceptance criteria

• New task created from the app appears in the SPA/Electron too (shared backend state)
• Sending a message to a busy task queues per current backend semantics; to a waiting task delivers immediately
• Interrupt (Esc-equivalent) works and reflects in state
• Send failures (disconnected) surface inline with retry
• Integration-tested against a live backend via a scripted flow

Scope & expectations

• Where: shared composer view, new-task sheet, ClaudiaKit send paths
• Out of scope: voice input (separate issue), slash-command palette

How to verify

UI — recording: create a task, exchange two messages, interrupt — all from the app, with the SPA visible showing the same task.