terminal_exit key.
Not a cognition.ai/ key. This follows the terminal-metadata
convention of third-party ACP agents (codex-acp, claude-agent-acp), which
the Windsurf chat client already parses (getTerminalExitMeta in
windsurf_acp). The chisel agent emits it on the final exec
tool_call_update so clients can render the exit code as command-card
chrome; terminal output itself still travels as content blocks.
Like [TerminalCwdMeta], this struct is designed to be merged standalone
into any tool call’s _meta via [Meta::merge_into].