[codex] Improve issue thread review flow (#4381)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Issue detail is where operators coordinate review, approvals, and
follow-up work with active runs
> - That thread UI needs to surface blockers, descendants, review
handoffs, and reply ergonomics clearly enough for humans to guide agent
work
> - Several small gaps in the issue-thread flow were making review and
navigation clunkier than necessary
> - This pull request improves the reply composer, descendant/blocker
presentation, interaction folding, and review-request handoff plumbing
together as one cohesive issue-thread workflow slice
> - The benefit is a cleaner operator review loop without changing the
broader task model

## What Changed

- restored and refined the floating reply composer behavior in the issue
thread
- folded expired confirmation interactions and improved post-submit
thread scrolling behavior
- surfaced descendant issue context and inline blocker/paused-assignee
notices on the issue detail view
- tightened large-board first paint behavior in `IssuesList`
- added loose review-request handoffs through the issue
execution-policy/update path and covered them with tests

## Verification

- `pnpm vitest run ui/src/pages/IssueDetail.test.tsx`
- `pnpm vitest run server/src/__tests__/issues-service.test.ts
server/src/__tests__/issue-execution-policy.test.ts`
- `pnpm exec vitest run --project @paperclipai/ui
ui/src/components/IssueChatThread.test.tsx
ui/src/components/IssueProperties.test.tsx
ui/src/components/IssuesList.test.tsx ui/src/lib/issue-tree.test.ts
ui/src/api/issues.test.ts`
- `pnpm exec vitest run --project @paperclipai/adapter-utils
packages/adapter-utils/src/server-utils.test.ts`
- `pnpm exec vitest run --project @paperclipai/server
server/src/__tests__/issue-comment-reopen-routes.test.ts -t "coerces
executor handoff patches into workflow-controlled review wakes|wakes the
return assignee with execution_changes_requested"`
- `pnpm exec vitest run --project @paperclipai/server
server/src/__tests__/issue-execution-policy.test.ts
server/src/__tests__/issues-service.test.ts`

## Visual Evidence

- UI layout changes are covered by the focused issue-thread component
and issue-detail tests listed above. Browser screenshots were not
attachable from this automated greploop environment, so reviewers should
use the running preview for final visual confirmation.

## Risks

- Moderate UI-flow risk: these changes touch the issue detail experience
in multiple spots, so regressions would most likely show up as
thread-layout quirks or incorrect review-handoff behavior

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex GPT-5-based coding agent with tool use and code execution
in the Codex CLI environment

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots or documented the visual verification path
- [ ] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/lib/issue-tree.ts

@@ -34,3 +34,39 @@ export function countDescendants(id: string, childMap: Map<string, Issue[]>): nu
   const children = childMap.get(id) ?? [];
   return children.reduce((sum, c) => sum + 1 + countDescendants(c.id, childMap), 0);
 }
+
+/**
+ * Filters a flat issue list to only descendants of `rootId`.
+ *
+ * This is intentionally useful even when the list contains unrelated issues:
+ * stale servers may ignore newer descendant-scoped query params, and the UI
+ * must still avoid rendering global issue data in a sub-issue panel.
+ */
+export function filterIssueDescendants(rootId: string, items: Issue[]): Issue[] {
+  const childrenByParentId = new Map<string, Issue[]>();
+  for (const item of items) {
+    if (!item.parentId) continue;
+    const siblings = childrenByParentId.get(item.parentId) ?? [];
+    siblings.push(item);
+    childrenByParentId.set(item.parentId, siblings);
+  }
+
+  const descendants: Issue[] = [];
+  const seen = new Set<string>([rootId]);
+  let frontier = [rootId];
+
+  while (frontier.length > 0) {
+    const nextFrontier: string[] = [];
+    for (const parentId of frontier) {
+      for (const child of childrenByParentId.get(parentId) ?? []) {
+        if (seen.has(child.id)) continue;
+        seen.add(child.id);
+        descendants.push(child);
+        nextFrontier.push(child.id);
+      }
+    }
+    frontier = nextFrontier;
+  }
+
+  return descendants;
+}