[codex] Improve issue detail and issue-list UX (#3678)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - A core part of that is the operator experience around reading issue
state, agent chat, and sub-task structure
> - The current branch had a long run of issue-detail and issue-list UX
fixes that all improve how humans follow and steer active work
> - Those changes mostly live in the UI/chat surface and should be
reviewed together instead of mixed with workspace/runtime work
> - This pull request packages the issue-detail, chat, markdown, and
sub-issue list improvements into one standalone change
> - The benefit is a cleaner, less jumpy, more reliable issue workflow
on desktop and mobile without coupling it to unrelated server/runtime
refactors

## What Changed

- Stabilized issue chat runtime wiring, optimistic comment handling,
queued-comment cancellation, and composer anchoring during live updates
- Fixed several issue-detail rendering and navigation regressions
including placeholder bleed, local polling scope, mobile inbox-to-issue
transitions, and visible refresh resets
- Improved markdown and rich-content handling with advisory image
normalization, editor fallback behavior, touch mention recovery, and
`issue:` quicklook links
- Refined sub-issue behavior with parent-derived defaults, current-user
inheritance fixes, empty-state cleanup, and a reusable issue-list
presentation for sub-issues
- Added targeted UI tests for the new issue-detail, chat scroll/message,
placeholder-data, markdown, and issue-list behaviors

## Verification

- `pnpm vitest run ui/src/components/IssueChatThread.test.tsx
ui/src/components/MarkdownEditor.test.tsx
ui/src/components/IssuesList.test.tsx
ui/src/context/LiveUpdatesProvider.test.tsx
ui/src/lib/issue-chat-messages.test.ts
ui/src/lib/issue-chat-scroll.test.ts
ui/src/lib/issue-detail-subissues.test.ts
ui/src/lib/query-placeholder-data.test.tsx
ui/src/hooks/usePaperclipIssueRuntime.test.tsx`

## Risks

- Medium: this branch touches the highest-traffic issue-detail UI paths,
so regressions would show up as chat/thread or sub-issue UX glitches
- The changes are UI-heavy and would benefit from reviewer screenshots
or a quick manual browser pass before merge

## Model Used

- OpenAI Codex coding agent (GPT-5-class runtime in Codex CLI; exact
deployed model ID is not exposed in this environment), reasoning
enabled, tool use and local code execution enabled

## 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 run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [ ] 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/components/CommentThread.tsx

@@ -134,6 +134,11 @@ function parseReassignment(target: string): CommentReassignment | null {
   return null;
 }
 
+function shouldImplicitlyReopenComment(issueStatus: string | undefined, assigneeValue: string) {
+  const isClosed = issueStatus === "done" || issueStatus === "cancelled";
+  return isClosed && assigneeValue.startsWith("agent:");
+}
+
 function humanizeValue(value: string | null): string {
   if (!value) return "None";
   return value.replace(/_/g, " ");
@@ -647,6 +652,7 @@ export function CommentThread({
   pendingApprovalAction = null,
   onVote,
   onAdd,
+  issueStatus,
   agentMap,
   currentUserId,
   imageUploadHandler,
@@ -663,7 +669,6 @@ export function CommentThread({
   composerDisabledReason = null,
 }: CommentThreadProps) {
   const [body, setBody] = useState("");
-  const [reopen, setReopen] = useState(true);
   const [submitting, setSubmitting] = useState(false);
   const [attaching, setAttaching] = useState(false);
   const effectiveSuggestedAssigneeValue = suggestedAssigneeValue ?? currentAssigneeValue;
@@ -784,14 +789,17 @@ export function CommentThread({
     if (!trimmed) return;
     const hasReassignment = enableReassign && reassignTarget !== currentAssigneeValue;
     const reassignment = hasReassignment ? parseReassignment(reassignTarget) : null;
+    const reopen = shouldImplicitlyReopenComment(
+      issueStatus,
+      hasReassignment ? reassignTarget : currentAssigneeValue,
+    ) ? true : undefined;
     const submittedBody = trimmed;
 
     setSubmitting(true);
     setBody("");
     try {
-      await onAdd(submittedBody, reopen ? true : undefined, reassignment ?? undefined);
+      await onAdd(submittedBody, reopen, reassignment ?? undefined);
       if (draftKey) clearDraft(draftKey);
-      setReopen(true);
       setReassignTarget(effectiveSuggestedAssigneeValue);
     } catch {
       setBody((current) =>
@@ -935,15 +943,6 @@ export function CommentThread({
                 </Button>
               </div>
             )}
-            <label className="flex items-center gap-1.5 text-xs text-muted-foreground cursor-pointer select-none">
-              <input
-                type="checkbox"
-                checked={reopen}
-                onChange={(e) => setReopen(e.target.checked)}
-                className="rounded border-border"
-              />
-              Re-open
-            </label>
             {enableReassign && reassignOptions.length > 0 && (
               <InlineEntitySelector
                 value={reassignTarget}