Praxis mode 5/8: Support praxis mode in agentop and the installer #1722

Part of #1714 · phase A, delivery step 5 · Section 4 (agentop, Release and install) · design in #1717 (docs/superpowers/specs/2026-11-07-praxis-mode-design.md)

Goal

Let a user turn on praxis mode, and keep it across updates.

Scope

• agentop reads praxis.enabled from GET /config: a fourth decoded field, documented in docs/proxy-contract.md.
• In praxis mode, agentop server …, the script commands and agentop tools scan --write refuse with a message that says why. One helper decides it for all of them.
• install.sh --praxis (refused on darwin/amd64). Setup adds the praxis block to the config, new or existing, and changes nothing else. The user's plugins and agentop server entries stay.
• To turn the mode off: praxis.enabled: false and a restart. The plugins run again, unchanged.
• agentop update keeps praxis mode, as it keeps --cpex. The supervisor refuses a gateway whose version differs from cortex.

Done when

• A fresh install.sh --praxis gives a working praxis mode.
• agentop update on a praxis install keeps it.
• make dev-install and agentop setup work without cortex-gateway while praxis.enabled is false.

Depends on

#1720 and #1721.

Assisted-By: Claude (Anthropic AI) noreply@anthropic.com