implement builtin.terminal — general-purpose shell pane

Flips the Tier-0 stub into a real extension. Contributes a Terminal
tab in the workspace slot that spawns $SHELL -l via IPC pane.spawn,
feeds pane.output events into xterm.dart's Terminal, and routes user
input back through pane.write. Viewport resize propagates via
pane.resize. Surface for disconnected-daemon / exited-shell states so
there's no silent dead tab.

Knows nothing about Claude deliberately — the Claude-specific pane
(primary-per-repo, tmux-backed, D-041) lives under builtin.claude in
the next steps.

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/kernel/src/i18n/catalog/builtin.terminal_en_us.json

@@ -0,0 +1,7 @@
+{
+  "tab.title": { "translation": "Terminal" },
+  "subtitle.spawning": { "translation": "spawning shell…" },
+  "subtitle.exited": { "translation": "Shell exited." },
+  "error.unavailable": { "translation": "Terminal unavailable" },
+  "error.daemon": { "translation": "Daemon not connected. Start `clide --daemon`." }
+}