Add session management tools for Copilot SDK

[maxtara]

Summary

Adds internal tools (registered via the Copilot SDK) to manage Cooper sessions. These tools allow the AI to create, list, and close session tabs directly.

Tools Added

• cooper_create_session: Create a new session tab with optional cwd, model, and initial prompt. Defaults to current session's folder and model. Opens in background (doesn't switch tabs).
• cooper_create_worktree_session: Create a git worktree and open a session in it. Validates that the path is a git repository. Use for parallel branch development.
• cooper_list_sessions: List all active session tabs with their ID, model, cwd, and active status.
• cooper_close_session: Close a session tab (cannot close the current session).
• cooper_get_current_session: Get info about the current session.
• cooper_get_models: List available AI models.
• cooper_get_favorite_models: Get user's favorite models.

Why Internal Tools vs MCP

Based on prior exploration, internal tools (via Copilot SDK) are better than MCP for actions that touch UI state:

• Tool responses flow through normal message handling - renderer reacts naturally via IPC events
• No need for complex IPC → custom event → renderer listener chains
• Tools run in main process with direct access to session state

Files Changed

• \src/main/sessionTools.ts\ - New file with tool definitions
• \src/main/main.ts\ - Integrate session tools into session creation
• \src/preload/preload.ts\ - Add IPC event listeners for session creation/close
• \src/renderer/App.tsx\ - Handle IPC events to create/close tabs in React state

Testing

• All 394 tests pass
• Build succeeds