20x: allow attaching files to messages in agent transcript

[dimavedenyapin]

Summary

• add per-message file attachments in desktop and mobile transcript composers
• pass attachment metadata through renderer/mobile API, IPC, and agent manager send paths
• include bounded attachment context in prompts (file refs + small text previews only) to avoid context bloat

Testing

• ELECTRON_RUN_AS_NODE=1 electron ./node_modules/vitest/vitest.mjs run src/renderer/src/hooks/use-agent-session.test.tsx src/renderer/src/components/tasks/TaskWorkspace.test.tsx src/mobile/components/ChatInput.test.tsx src/main/agent-manager.test.ts
• pnpm test:run
• pnpm typecheck