Summary

Adds a "New Chat" button to the app builder chat pane header, allowing users to start a fresh AI session within an existing project. The new session gets a clean LLM context while preserving the project's full codebase and previous session history.

User flow:

1. User clicks the SquarePen icon in the chat header (disabled while streaming)
2. The chat area clears to show "New chat session — Your code is preserved — the AI starts with fresh context" and the input is enabled
3. Clicking the button again cancels and returns to the current session view
4. Sending a message creates a new cloud agent session via forceNewSession: true on the existing sendMessage endpoint; the old session collapses into an expandable history block via the existing session-change machinery

Architecture: The forceNewSession flag is handled at the ProjectManager level — it bypasses the session/streaming layers entirely and calls tRPC directly, then delegates to the existing handleSessionChanged handler. Zero changes to V1/V2 session or streaming code. No DB migration needed (the reason column is freeform text).

Verification

• Manual code review of the full change set; all 10 files verified for type consistency
• Traced tRPC input flow from Zod schema → service type → service logic → both personal and org routers
• Confirmed forceNewSession path in app-builder-service.ts bypasses shouldCreateNewSession and uses the existing createV1Session/createV2Session helpers with the new user_initiated reason
• Confirmed pendingNewSession state resets correctly on send, cancel, and session creation

Visual Changes

N/A — chat pane only; a SquarePen icon is added to the header bar.

Reviewer Notes

• The SquarePen button is a toggle: click to enter pending mode, click again to cancel. It is disabled while isStreaming is true so users cannot trigger it mid-stream.
• The sendMessageAsNewSession function in ProjectManager.ts calls tRPC directly (rather than going through the session layer) because the existing session must be ended server-side before the new one is created. The response triggers handleSessionChanged, which reuses all existing session-change UI logic.
• AppBuilderSessionReason.UserInitiated ('user_initiated') is added to the schema constant but requires no migration since the column is text NOT NULL with no DB-level constraint on values.