fix forked prompt attachments losing file parts

[kitlangton]

Summary

• fix a TUI bug where forking or reusing a prompt with a file or image attachment could silently lose that attachment on the new message
• strip persisted part ids when rehydrating saved file parts into prompt state so forked prompts do not keep pointing at the old message row
• assign fresh part ids on submit and cover the stale-id path with a focused regression test

Testing

• bun run test test/cli/cmd/tui/prompt-part.test.ts
• bun run test test/session/prompt.test.ts
• bun typecheck