Add Muse Code as a default coding agent (#9915)

* Add Muse Code as a default coding agent

Meta ships Muse Code only as a binary, so it installs from the AUR
(muse-code-bin) instead of mise, and a fresh install runs the muse login
browser flow in the install terminal before the agent opens.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0148qKzr366p2Ubu2igCLvPg

* Refine Muse Code menu and prompt forwarding

* Install Muse Code from OPR

* Install Muse Code through mise's HTTP backend

* Preinstall the Muse mise stub

* Use the shared Muse installation flow

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: David Heinemeier Hansson <david@hey.com>
(cherry picked from commit 0d223fe820)

default/omarchy/omarchy-menu.jsonc

@@ -138,6 +138,7 @@
   "setup.default.agent.gemini": {"icon":"󰫢","label":"Gemini","checked":"[[ \"$(omarchy-default-agent)\" == \"gemini\" ]]","action":"omarchy-default-agent gemini"},
   "setup.default.agent.grok": {"icon":"","iconFont":"omarchy","label":"Grok","checked":"[[ \"$(omarchy-default-agent)\" == \"grok\" ]]","action":"omarchy-default-agent grok"},
   "setup.default.agent.hermes": {"icon":"","iconFont":"omarchy","label":"Hermes","checked":"[[ \"$(omarchy-default-agent)\" == \"hermes\" ]]","action":"omarchy-default-agent hermes"},
+  "setup.default.agent.muse": {"icon":"󰛤","label":"Muse Code","checked":"[[ \"$(omarchy-default-agent)\" == \"muse\" ]]","action":"omarchy-default-agent muse"},
   "setup.default.agent.omp": {"icon":"","iconFont":"omarchy","label":"omp","checked":"[[ \"$(omarchy-default-agent)\" == \"omp\" ]]","action":"omarchy-default-agent omp"},
   "setup.default.agent.openclaw": {"icon":"","iconFont":"omarchy","label":"OpenClaw","checked":"[[ \"$(omarchy-default-agent)\" == \"openclaw\" ]]","action":"omarchy-default-agent openclaw"},
   "setup.default.agent.opencode": {"icon":"","iconFont":"omarchy","label":"OpenCode","checked":"[[ \"$(omarchy-default-agent)\" == \"opencode\" ]]","action":"omarchy-default-agent opencode"},