Match ACP session modes to the CLI's permission modes - #1189

- ACP sessions offer the CLI's permission modes, auto, ask, and
  full-access, named Auto, Ask, and Full access, in both the mode config
  option and the modes field. The code mode is removed.
- A session starts in the saved permission_mode, or auto when none is
  saved, and reports the mode it enforces. Sessions previously reported
  ask while enforcing the saved mode.
- Choosing a mode with session/set_config_option or session/set_mode
  saves it as the default permission mode, like /permissions, so new
  sessions and the CLI start in it.
- A session reads the saved mode when it starts, so a long-running ACP
  process follows a mode another fx process saved. Open sessions keep
  their own mode.
- In full-access, tool calls run without a permission request or an
  automatic review.
- An unknown mode, including code, returns an "Unknown mode" error and
  changes nothing. session/set_mode without a modeId, and a mode change
  without an active session, also return errors.
- Mode option values report their permission mode in
  _meta.fx.permissionMode instead of a top-level field outside ACP's
  schema.
- libfx host config saves modes as auto, ask, and full-access, and
  ignores a saved code when restoring.