Offer T3 Code as the preferred agent, opening a thread in the Panama project, and install it with the desktop

tests/quickshell/panama-agent-contract

@@ -27,8 +27,11 @@
 #   6. panama-agent-crash carries all four coredump facts and the absolute path
 #      to the skill. The path is what makes the ladder work for an agent whose
 #      harness has no skill mechanism.
+#   7. T3 Code opens no terminal. It starts a thread in the checkout's project
+#      with the prompt intact on stdin, and opens that thread's t3code:// link.
 #
-# Hermetic on purpose: kitty, claude, codex, setsid and coredumpctl are all
+# Hermetic on purpose: kitty, claude, codex, t3, xdg-open, setsid and
+# coredumpctl are all
 # stubbed onto PATH and the settings file is fabricated, so no agent is ever
 # launched and no window is ever opened. This is NOT in tests/desktop-hijacking
 # for exactly that reason -- it must stay runnable mid-session.
@@ -79,6 +82,23 @@ for agent in claude codex; do
 STUB
 done
 
+# T3 Code's CLI. It records each call and its stdin, and answers `thread start`
+# with a thread id the way the real one does.
+cat >"$stub/t3" <<STUB
+#!/usr/bin/env bash
+printf '%s\0' "\$@" >>"$work/t3-calls"
+printf '\n' >>"$work/t3-calls"
+if [[ "\$1 \$2" == "thread start" ]]; then
+    cat >"$work/t3-stdin"
+    printf 'thread-123\n'
+fi
+STUB
+
+cat >"$stub/xdg-open" <<STUB
+#!/usr/bin/env bash
+printf '%s' "\$1" >"$work/opened"
+STUB
+
 # A hand-run PID has no core here; the real one would be a slow lookup against
 # this machine's journal.
 cat >"$stub/coredumpctl" <<'STUB'
@@ -104,7 +124,9 @@ chmod +x "$stub"/*
 # from the previous run are cleared first, so "was not launched" is a missing
 # file rather than a stale one.
 launch() {
-    rm -f "$work/argv" "$work/cwd" "$work/setsid-used" "$work/agent-actually-ran"
+    rm -f "$work/argv" "$work/cwd" "$work/setsid-used" "$work/agent-actually-ran" \
+        "$work/t3-calls" "$work/t3-stdin" "$work/opened"
+    T3CODE_HOME="$work/t3home" \
     PATH="$stub:/usr/bin:/bin" \
         PANAMA_PATH="$repo_dir" \
         PANAMA_AGENT_SETTINGS="$settings" \
@@ -252,6 +274,35 @@ else
     note 'launching codex with auto-approve off opened no terminal'
 fi
 
+# ── 7. T3 Code ──────────────────────────────────────────────────────────────
+
+# The recorded t3 calls, one per line, arguments joined by \x1f.
+t3_calls() { tr '\0' '\037' <"$work/t3-calls" 2>/dev/null; }
+
+mkdir -p "$work/t3home/userdata"
+printf 'env-456' >"$work/t3home/userdata/environment-id"
+
+printf '{"preferredAgent":"t3","agentAutoApprove":true}\n' >"$settings"
+dash_prompt=$'--looks-like-a-flag\nsecond line'
+launch "$launcher" --prompt "$dash_prompt"
+status=$?
+
+(( status == 0 )) || note "panama-agent exited $status launching T3 Code: $(head -1 "$work/stderr")"
+[[ -e "$work/argv" ]] && note 'T3 Code was launched in a terminal; it should open a thread in the app'
+t3_calls | grep -qxF "thread"$'\x1f'"start"$'\x1f'"$repo_dir"$'\x1f'"-"$'\x1f' \
+    || note 'T3 Code was not asked to start a thread in the checkout with the prompt on stdin'
+[[ "$(cat "$work/t3-stdin" 2>/dev/null)" == "$dash_prompt" ]] \
+    || note 'the T3 thread did not receive the prompt intact on stdin'
+[[ "$(cat "$work/opened" 2>/dev/null)" == "t3code://threads/env-456/thread-123" ]] \
+    || note "T3 Code opened '$(cat "$work/opened" 2>/dev/null)' rather than the new thread's link"
+
+# No prompt means nothing to start: the app opens on the project.
+launch "$launcher"
+t3_calls | grep -qF "thread"$'\x1f'"start" \
+    && note 'with no prompt T3 Code still started a thread'
+t3_calls | grep -qxF "app"$'\x1f'"$repo_dir"$'\x1f' \
+    || note 'with no prompt T3 Code was not opened on the checkout'
+
 # ── An agent installed where only the user's own PATH looks ─────────────────
 #
 # Both agents install themselves into ~/.local/bin. The rungs are clicked from