fix(prompter): MegaTask review card scrolls + confirm-batch guard releases on failure (#376)

The MegaTask review card was a static sibling of the scrollable chat list with
no height bound, so a tall batch overflowed the clipped container and stranded
the launch buttons off-screen with no scrollbar. It now owns the scroll area
(min-h-0 flex-1 overflow-y-auto), matching the pattern ChatMessages already uses.

confirm_live_batch's Redis idempotency guard (1h TTL) was acquired before the
build but only released via a success sidecar, so a build failure wedged the
session: every retry hit ServiceError("already in progress") -> HTTP 500 for an
hour. The build now releases the guard on any failure before the sidecar write,
so a retry re-attempts (and surfaces the real error) instead of being locked out.
Extracted _build_confirm_batch to keep the try/except thin.

Adds DB-backed tests for the panel the_work[].project_id shape, multi-cell
root-subtasks, dense same-repo collisions (both routes), and guard release.

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

panel/src/app/(dashboard)/prompter/page.tsx

@@ -152,18 +152,13 @@ export default function PrompterPage() {
                 )}
               </div>
             </div>
-          ) : (
-            <ChatMessages
-              messages={messages}
-              onStart={launchTask}
-              onKeepChatting={keepChatting}
-              isLaunching={isLaunching}
-            />
-          )}
-
-          {/* MegaTask review — the agent proposed a batch; confirm them together */}
-          {state === "batch_preview" && batch && (
-            <div className="mx-4 mb-2">
+          ) : state === "batch_preview" && batch ? (
+            /* MegaTask review — the agent proposed a batch; confirm them
+               together. Owns the scroll area (min-h-0 + overflow-y-auto) so a
+               tall batch — many tasks, per-cell pickers, the wave plan — scrolls
+               instead of overflowing the clipped parent and stranding the launch
+               buttons off-screen. */
+            <div className="min-h-0 flex-1 overflow-y-auto px-4 py-3">
               <BatchReviewCard
                 batch={batch}
                 waves={batchWaves}
@@ -174,6 +169,13 @@ export default function PrompterPage() {
                 isLaunching={isLaunching}
               />
             </div>
+          ) : (
+            <ChatMessages
+              messages={messages}
+              onStart={launchTask}
+              onKeepChatting={keepChatting}
+              isLaunching={isLaunching}
+            />
           )}
 
           {/* Live activity indicator — "watch it work" (prominent) */}