Introduces a `/new` slash command that lets users create a fresh chat
session for the current agent without leaving the chat input.
- Add `createNewSession` to `useGateway` that calls `sessions.create`
on the gateway (with a client-side fallback key when the RPC is
unavailable).
- Register `/new` in the slash-command menu with i18n descriptions
across all 8 supported languages.
- Wire `onNewSession` through `Chat` → `ChatInput` so typing `/new`
triggers session creation.
- Add `extractAgentIdFromKey` and `formatAgentId` helpers to
`sessionName.ts` to derive a human-readable agent name from the
session key (e.g. `agent:my-cool-bot:…` → "My Cool Bot").
- Use the new helpers in `Header` and `App` to show per-session agent
names, especially for sub-agent sessions where the gateway-level
identity differs from the session agent.
Made-with: Cursor
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