Host shell discovery

The process host capability owns shell discovery and shell-mode invocation. This keeps IDEs, TUIs, headless CLI runs, and cloud workers on the same contract instead of hardcoding /bin/sh, cmd, or host-specific settings in each integration.

• process.list_shells returns {shells, default_shell_id}. Each shell entry has id, label, path, platform, available, supports_login, supports_interactive, default_args, login_args, and source.
• process.get_default_shell returns the selected shell object for the current host/session.
• process.set_default_shell may be implemented by stateful hosts. Harn's standalone fallback stores the selection for the current thread/session.
• process.shell_invocation resolves {shell_id?, shell?, command?, login?, interactive?} into {program, args, command_arg_index, shell}. When neither shell_id nor shell is supplied, it uses the selected default shell.

Shell-mode command runners may pass a shell object or shell ID resolved through this capability, and otherwise use the selected default shell. argv mode remains preferred for programmatic execution; shell mode is for user-authored commands and interactive shell semantics. The normative schema is spec/schemas/host-shell-discovery.schema.json.