[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/lib/optimistic-issue-comments.ts

@@ -96,6 +96,21 @@ export function mergeIssueComments(
   return sortIssueComments(merged);
 }
 
+export function takeOptimisticIssueComment(
+  comments: OptimisticIssueComment[],
+  clientId: string,
+): { comments: OptimisticIssueComment[]; comment: OptimisticIssueComment | null } {
+  const index = comments.findIndex((comment) => comment.clientId === clientId);
+  if (index === -1) {
+    return { comments, comment: null };
+  }
+
+  return {
+    comments: comments.filter((comment) => comment.clientId !== clientId),
+    comment: comments[index] ?? null,
+  };
+}
+
 export function flattenIssueCommentPages(
   pages: ReadonlyArray<ReadonlyArray<IssueComment>> | undefined,
 ): IssueComment[] {
@@ -254,3 +269,16 @@ export function upsertIssueCommentInPages(
   nextPages[0] = sortIssueCommentsDesc([...nextPages[0]!, nextComment]);
   return nextPages;
 }
+
+export function removeIssueCommentFromPages(
+  pages: ReadonlyArray<ReadonlyArray<IssueComment>> | undefined,
+  commentId: string,
+): IssueComment[][] {
+  if (!pages || pages.length === 0) {
+    return [];
+  }
+
+  return pages
+    .map((page) => page.filter((comment) => comment.id !== commentId))
+    .filter((page) => page.length > 0);
+}