feat: add sandbox selection and visual selection sending to Codex sessions

[lukeraphael]

This PR adds two launcher improvements to codex.nvim:

• Support for configuring Codex sandbox mode in plugin setup via sandbox = 'read-only' | 'workspace-write' | 'danger-full-access'
• Support for overriding sandbox mode per launch with :Codex and :CodexToggle

It also adds a new visual-selection workflow:

• :CodexSendSelection and optional keymaps.send_selection now send highlighted text to Codex
• If a Codex chat is already active, the selected text is sent into that existing session and the window is focused
• If no chat is active, the selection starts a new Codex session as the initial prompt

Implementation notes

• Added sandbox validation and CLI argument assembly for --sandbox
• Added prompt forwarding so selected text can be passed as the initial Codex input
• Added selection extraction from the current visual range
• Reuse of active Codex sessions is implemented with chansend() for terminal-backed sessions

Docs

• Updated README with sandbox config/usage
• Documented visual selection sending and example visual-mode mapping

Testing

Added specs for:

• default sandbox config
• per-launch sandbox override
• prompt forwarding to Codex
• sending visual selection as a new prompt
• reusing an active Codex session for selection sending