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)

test/shell.d/menu-test.sh

@@ -207,7 +207,9 @@ const expectedAgents = {
   openclaw: { icon: '\ue90c', iconFont: 'omarchy', label: 'OpenClaw' },
   copilot: { icon: '', label: 'Copilot' },
   crush: { icon: '󰋑', label: 'Crush' },
+  muse: { icon: '󰛤', label: 'Muse Code' },
   'cursor-agent': { icon: '\ue90d', iconFont: 'omarchy', label: 'Cursor CLI' },
+
 }
 assert(
   Object.entries(expectedAgents).every(([agent, expected]) => {
@@ -220,13 +222,13 @@ assert(
       && !entry.when
       && entry.checked.includes(`== \"${agent}\"`)
   }),
-  'menu exposes every mise-installable coding agent with its own glyph under Defaults > Agent'
+  'menu exposes every supported coding agent with its own glyph under Defaults > Agent'
 )
 assertDeepEqual(
   defaultItems
     .filter(item => item.parent === 'setup.default.agent')
     .map(item => item.label),
-  ['Claude', 'Codex', 'Copilot', 'Crush', 'Cursor CLI', 'Gemini', 'Grok', 'Hermes', 'omp', 'OpenClaw', 'OpenCode', 'Pi'],
+  ['Claude', 'Codex', 'Copilot', 'Crush', 'Cursor CLI', 'Gemini', 'Grok', 'Hermes', 'Muse Code', 'omp', 'OpenClaw', 'OpenCode', 'Pi'],
   'menu sorts coding agents alphabetically'
 )
 assert(!defaultById['install.ai.crush'], 'menu removes Crush from Install > AI')