Support `--resume latest` to directly resume most recent session

Summary

Add support for claude --resume latest (or claude -r latest) to directly resume the most recent session without going through the interactive picker.

Current behavior

• claude --resume — opens an interactive session picker where you must scroll/select and press Enter
• claude --resume <session-id> — resumes a specific session by its ID
• claude --continue — resumes the most recent conversation, but scoped to the current working directory

Proposed behavior

• claude --resume latest — directly resumes the most recent session globally (across all directories), skipping the interactive picker

When the value latest is passed to --resume, the CLI should resolve it to the most recent session (by timestamp) and resume it immediately, the same way --resume <session-id> does with a specific ID.

Use case

This is a common workflow shortcut. Users frequently want to jump back into their last conversation without navigating the picker. --continue partially addresses this but is scoped to the current directory, so it won't find sessions started in other directories.