refactor: address review feedback on the pinned PlanCard

Address code review feedback on cacdec3 (pin the latest ACP plan above
the chat transcript):

- SessionChatPane.svelte: key the PlanCard on session?.id so the
  expanded/collapsed toggle resets when the pane is reused for a
  different session, letting defaultExpanded apply per session instead
  of a collapse in one session leaking into the next.
- acpTranscript.ts: drop the redundant Array.isArray(rawEntries)
  re-check in latestPlan since arrayProp already returns
  unknown[] | null; use a plain null check to match
  latestAvailableCommands.
- acpTranscript.ts / acpTranscript.test.ts: remove the unused priority
  field from PlanEntry, its parsing, and the test assertions, since
  PlanCard never renders it. One raw input in the tests keeps a
  priority prop to cover that unparsed props are dropped.

Verified with vitest (682 passing) and svelte-check (0 errors).

Signed-off-by: Matt Toohey <contact@matttoohey.com>

Author: matt2e

apps/staged/src/lib/features/sessions/SessionChatPane.svelte

@@ -1737,9 +1737,13 @@
   bind:this={modalElement}
   class={`session-chat-pane ${compact ? 'compact' : ''} ${dragOver ? 'drag-over' : ''}`}
 >
-  <!-- Latest plan, pinned above the scrollable transcript -->
+  <!-- Latest plan, pinned above the scrollable transcript. Keyed on the
+       session so the expanded/collapsed toggle resets when this pane is
+       reused for a different session. -->
   {#if plan}
-    <PlanCard entries={plan} defaultExpanded={!viewport.isMobile && !compact} />
+    {#key session?.id}
+      <PlanCard entries={plan} defaultExpanded={!viewport.isMobile && !compact} />
+    {/key}
   {/if}
 
   <!-- Messages area -->

apps/staged/src/lib/features/sessions/acpTranscript.test.ts

@@ -718,8 +718,8 @@ describe('latestPlan', () => {
     ]);
 
     expect(plan).toEqual([
-      { content: 'Check UI', status: 'completed', priority: 'high' },
-      { content: 'Fix bug', status: 'in_progress', priority: null },
+      { content: 'Check UI', status: 'completed' },
+      { content: 'Fix bug', status: 'in_progress' },
     ]);
   });
 
@@ -766,9 +766,9 @@ describe('latestPlan', () => {
     ]);
 
     expect(plan).toEqual([
-      { content: 'Mystery step', status: 'pending', priority: null },
-      { content: 'Statusless step', status: 'pending', priority: null },
-      { content: 'Failed step', status: 'failed', priority: null },
+      { content: 'Mystery step', status: 'pending' },
+      { content: 'Statusless step', status: 'pending' },
+      { content: 'Failed step', status: 'failed' },
     ]);
   });
 });

apps/staged/src/lib/features/sessions/acpTranscript.ts

@@ -278,7 +278,6 @@ export type PlanEntryStatus = 'pending' | 'in_progress' | 'completed' | 'failed'
 export interface PlanEntry {
   content: string;
   status: PlanEntryStatus;
-  priority: string | null;
 }
 
 /**
@@ -293,7 +292,7 @@ export function latestPlan(metadataMessages: SessionMessage[]): PlanEntry[] | nu
     .find((message) => message.acpEventKind === 'plan_update');
   if (!latest) return null;
   const rawEntries = arrayProp(latest.acpContent, 'entries');
-  if (!Array.isArray(rawEntries)) return null;
+  if (!rawEntries) return null;
 
   const entries = rawEntries
     .map((entry) => {
@@ -302,7 +301,6 @@ export function latestPlan(metadataMessages: SessionMessage[]): PlanEntry[] | nu
       return {
         content,
         status: normalizePlanEntryStatus(stringProp(entry, 'status')),
-        priority: stringProp(entry, 'priority'),
       };
     })
     .filter((entry): entry is PlanEntry => entry !== null);