Add OpenCode 2 (opencode2) as a choice under Setup > Defaults > Agent

[wojtekpiskorz]

Title: Add OpenCode 2 (opencode2) as a choice under Setup > Defaults > Agent

OpenCode 2.0 (currently in beta) installs as a separate binary, opencode2, and is
designed to run side by side with v1 (opencode). Omarchy's default-agent picker
currently hardcodes the agent list in three places:

• bin/omarchy-default-agent (selection + mise install)
• bin/omarchy-agent (launcher flags; unknown agent → "Unsupported default agent")
• default/omarchy/omarchy-menu.jsonc (setup.default.agent.* rows)

So there is no supported way to make the Agent keybinding / omarchy agent open
OpenCode 2 — writing opencode2 to ~/.config/omarchy/defaults/agent manually results
in "Unsupported default agent: opencode2".

Proposal: add an opencode2 case:

# omarchy-default-agent
opencode2) agent="opencode2"; name="OpenCode 2"; agent_package="..." ;;

# omarchy-agent
opencode2)
  command=(opencode2 --auto)   # --auto = auto-approve permissions, same semantic as v1
  [[ -n ${prompt:-} ]] && command+=(--prompt "$prompt")
  ;;

# omarchy-menu.jsonc
"setup.default.agent.opencode2": {"icon":"","iconFont":"omarchy","label":"OpenCode 2","checked":"[[ \"$(omarchy-default-agent)\" == \"opencode2\" ]]","action":"omarchy-default-agent opencode2"},

Install-channel caveat (the actual design decision): OpenCode 2 is currently
published only on npm dist-tags (@opencode-ai/cli@next / @beta, needs the postinstall
script allowed), and mise's npm: backend can't resolve dist-tags — so
agent_package="npm:@opencode-ai/cli@next" won't work as-is. Options: pin a beta
version via mise, install via npm -g in the case handler, or wait for the stable
channel / mise support.

Workaround in use meanwhile (user-space, update-safe): menu row extension in
~/.config/omarchy/extensions/omarchy-menu.jsonc + two small scripts in ~/.local/bin
(a selector that installs via npm and writes defaults/agent, and a keybinding
dispatcher that intercepts opencode2 and delegates everything else to the packaged
omarchy-agent). Happy to turn this into a PR if you want it before the stable release.

System: Omarchy 4.0.1-1, opencode2 v0.0.0-beta-17823

[infosec-us-team]

OpenCode 1 and OpenCode 2 both use the opencode command and are no longer installed side by side.

https://opencode.ai/v2/docs/migrate-v1/

We should migrate the default to OpenCode v2.