Reshape the agent launcher into omarchy agent (#6757)

* Reshape the agent launcher into omarchy agent

omarchy-launch-agent becomes omarchy-agent, with prompts on omarchy-agent-prompt
rather than the bare route: `omarchy agent` is both a command and a group, so a
positional prompt there would shadow any subcommand under it. The launcher takes
flags only and points at `omarchy agent prompt` when handed one.

Every agent window now launches under a fixed org.omarchy.agent app-id instead of
omarchy-launch-tui's default of org.omarchy.<binary>, so one rule floats them all
whichever agent is default.

Omarchy also stops picking an agent for you. omarchy-default-agent prints nothing
until one is chosen, leaving every entry under Setup > Defaults > Agent unchecked,
and a first-run invitation offers to take you there.

* Wordsmith

* Cover the agent routes and the invitation

The route split is the point of the change, so exercise `omarchy agent`,
`omarchy agent prompt`, and a rejected positional prompt through the router
rather than only the binaries behind them.

The invitation gets the same treatment as the Voxtype and fingerprint ones: it
notifies once, opens the agent defaults menu, and leaves both the notification
and the marker alone for anyone who already chose an agent.

* Offer the agent choice from the keybinding

Super + Shift + Ctrl + A now runs `omarchy-agent --pick`, which opens Setup >
Defaults > Agent when nothing is chosen yet. A keypress that writes to stderr
and opens nothing just looks broken.

* Reach existing installs with the agent invitation

first-run installs the invitation hook, and existing accounts marked it complete
long ago, so they would never see it -- while being the accounts most likely to
need it, since the old getter returned opencode implicitly and most have no
agent recorded at all. Post-update hooks run later in the same update, so the
invitation arrives without waiting for another one.

* Say what the Defaults submenus set

Setup > Defaults lists Agent, Browser, Terminal, Editor, but the header inside
each repeated the same bare word, which reads as a category rather than a
setting -- and says nothing at all when the menu is summoned straight into it.
The list keeps its short labels; the headers now name the setting.

migrations/1786549201.sh

@@ -0,0 +1,10 @@
+echo "Invite existing installs to pick a default agent"
+
+# The invitation is installed by first-run, which existing accounts have already
+# marked complete, so they would never receive it. They are also the accounts
+# most likely to need it: the old getter returned opencode implicitly, so most
+# have no agent recorded at all.
+#
+# Post-update hooks run later in this same update, so the invitation appears
+# without waiting for another one. The hook decides whether to notify.
+omarchy-hook-install post-update "$OMARCHY_PATH/install/user/first-run/setup-agent.hook"