[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/issue-chat-messages.test.ts

@@ -3,6 +3,7 @@ import type { Agent } from "@paperclipai/shared";
 import {
   buildAssistantPartsFromTranscript,
   buildIssueChatMessages,
+  stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
 } from "./issue-chat-messages";
@@ -527,3 +528,69 @@ describe("buildIssueChatMessages", () => {
     });
   });
 });
+
+describe("stabilizeThreadMessages", () => {
+  it("reuses unchanged message objects across rebuilds", () => {
+    const firstPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const firstStable = stabilizeThreadMessages(firstPass, [], new Map());
+    const secondPass = buildIssueChatMessages({
+      comments: [
+        createComment(),
+        createComment({
+          id: "comment-2",
+          body: "New message",
+          createdAt: new Date("2026-04-06T12:01:00.000Z"),
+          updatedAt: new Date("2026-04-06T12:01:00.000Z"),
+        }),
+      ],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const secondStable = stabilizeThreadMessages(
+      secondPass,
+      firstStable.messages,
+      firstStable.cache,
+    );
+
+    expect(secondStable.messages).toHaveLength(2);
+    expect(secondStable.messages[0]).toBe(firstStable.messages[0]);
+    expect(secondStable.messages[1]?.id).toBe("comment-2");
+  });
+
+  it("reuses the previous array when nothing semantically changed", () => {
+    const firstPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const firstStable = stabilizeThreadMessages(firstPass, [], new Map());
+    const secondPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const secondStable = stabilizeThreadMessages(
+      secondPass,
+      firstStable.messages,
+      firstStable.cache,
+    );
+
+    expect(secondStable.messages).toBe(firstStable.messages);
+  });
+});