feat: add integrated terminal backend

- add a PTY manager with platform shell resolution and streamed data/exit events
- expose terminal lifecycle and terminal height IPC through preload and app service
- persist terminal panel height in workspace settings and document the backend contract
- add backend tests and validate native packaging with bun run package

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/channels.ts

@@ -14,10 +14,17 @@ export const ipcChannels = {
   deletePattern: 'patterns:delete',
   setPatternFavorite: 'patterns:set-favorite',
   setTheme: 'settings:set-theme',
+  setTerminalHeight: 'settings:set-terminal-height',
   saveMcpServer: 'tooling:mcp:save',
   deleteMcpServer: 'tooling:mcp:delete',
   saveLspProfile: 'tooling:lsp:save',
   deleteLspProfile: 'tooling:lsp:delete',
+  describeTerminal: 'terminal:describe',
+  createTerminal: 'terminal:create',
+  restartTerminal: 'terminal:restart',
+  writeTerminal: 'terminal:write',
+  resizeTerminal: 'terminal:resize',
+  killTerminal: 'terminal:kill',
   updateSessionTooling: 'sessions:update-tooling',
   updateSessionApprovalSettings: 'sessions:update-approval-settings',
   createSession: 'sessions:create',
@@ -41,6 +48,8 @@ export const ipcChannels = {
   selectSession: 'selection:session',
   openAppDataFolder: 'troubleshooting:open-app-data-folder',
   resetLocalWorkspace: 'troubleshooting:reset-local-workspace',
+  terminalData: 'terminal:data',
+  terminalExit: 'terminal:exit',
   workspaceUpdated: 'workspace:updated',
   sessionEvent: 'sessions:event',
 } as const;