feat(maestro): collapse the spawn form worktree section by default (#93)

Closed by default behind a disclosure reusing the sidebar folder-header
affordance (catalog.ts), since most spawns take the default target and
the section was the largest block in the form. The collapsed header
states whether a worktree will be created, so #91's off-by-default for
a worktree row stays visible without expanding.

Fixes a latent [hidden] + display:flex bug for the new .wt-collapsible
element (author CSS beats the UA [hidden] rule at equal specificity);
.wt-body has the same bug pre-existing and is out of scope here.

web/src/styles.css

@@ -1286,18 +1286,33 @@ textarea {
 .worktree {
   border: 1px solid var(--border);
   border-radius: var(--r-2);
-  padding: var(--sp-3);
+  padding: 0 var(--sp-3);
   margin: 0;
   display: flex;
   flex-direction: column;
-  gap: var(--sp-3);
   background: color-mix(in oklab, var(--surface-2) 50%, transparent);
 }
 
-.worktree legend {
-  padding: 0 6px;
-  color: var(--ink);
-  font-size: var(--fs-13);
+.worktree .wt-toggle {
+  margin: 0 calc(var(--sp-3) * -1);
+  padding-left: var(--sp-3);
+  width: calc(100% + var(--sp-3) * 2);
+}
+
+.wt-summary {
+  color: var(--muted);
+  font-size: var(--fs-12);
+}
+
+.wt-collapsible {
+  display: flex;
+  flex-direction: column;
+  gap: var(--sp-3);
+  padding-bottom: var(--sp-3);
+}
+
+.wt-collapsible[hidden] {
+  display: none;
 }
 
 .form-error {