[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>

server/src/__tests__/issue-comment-reopen-routes.test.ts

@@ -225,6 +225,40 @@ describe("issue comment reopen routes", () => {
     );
   });
 
+  it("implicitly reopens closed issues via the PATCH comment path when reassigning to an agent", async () => {
+    mockIssueService.getById.mockResolvedValue(makeIssue("done"));
+    mockIssueService.update.mockImplementation(async (_id: string, patch: Record<string, unknown>) => ({
+      ...makeIssue("done"),
+      ...patch,
+    }));
+
+    const res = await request(await installActor(createApp()))
+      .patch("/api/issues/11111111-1111-4111-8111-111111111111")
+      .send({ comment: "hello", assigneeAgentId: "33333333-3333-4333-8333-333333333333" });
+
+    expect(res.status).toBe(200);
+    expect(mockIssueService.update).toHaveBeenCalledWith(
+      "11111111-1111-4111-8111-111111111111",
+      expect.objectContaining({
+        assigneeAgentId: "33333333-3333-4333-8333-333333333333",
+        status: "todo",
+        actorAgentId: null,
+        actorUserId: "local-board",
+      }),
+    );
+    expect(mockLogActivity).toHaveBeenCalledWith(
+      expect.anything(),
+      expect.objectContaining({
+        action: "issue.updated",
+        details: expect.objectContaining({
+          reopened: true,
+          reopenedFrom: "done",
+          status: "todo",
+        }),
+      }),
+    );
+  });
+
   it("reopens closed issues via the PATCH comment path", async () => {
     mockIssueService.getById.mockResolvedValue(makeIssue("done"));
     mockIssueService.update.mockImplementation(async (_id: string, patch: Record<string, unknown>) => ({
@@ -259,6 +293,48 @@ describe("issue comment reopen routes", () => {
     );
   });
 
+  it("implicitly reopens closed issues via POST comments when an agent is assigned", async () => {
+    mockIssueService.getById.mockResolvedValue(makeIssue("done"));
+    mockIssueService.update.mockImplementation(async (_id: string, patch: Record<string, unknown>) => ({
+      ...makeIssue("done"),
+      ...patch,
+    }));
+
+    const res = await request(await installActor(createApp()))
+      .post("/api/issues/11111111-1111-4111-8111-111111111111/comments")
+      .send({ body: "hello" });
+
+    expect(res.status).toBe(201);
+    expect(mockIssueService.update).toHaveBeenCalledWith(
+      "11111111-1111-4111-8111-111111111111",
+      { status: "todo" },
+    );
+    expect(mockHeartbeatService.wakeup).toHaveBeenCalledWith(
+      "22222222-2222-4222-8222-222222222222",
+      expect.objectContaining({
+        reason: "issue_reopened_via_comment",
+        payload: expect.objectContaining({
+          reopenedFrom: "done",
+        }),
+      }),
+    );
+  });
+
+  it("does not implicitly reopen closed issues via POST comments when no agent is assigned", async () => {
+    mockIssueService.getById.mockResolvedValue({
+      ...makeIssue("done"),
+      assigneeAgentId: null,
+      assigneeUserId: "local-board",
+    });
+
+    const res = await request(await installActor(createApp()))
+      .post("/api/issues/11111111-1111-4111-8111-111111111111/comments")
+      .send({ body: "hello" });
+
+    expect(res.status).toBe(201);
+    expect(mockIssueService.update).not.toHaveBeenCalled();
+  });
+
   it("interrupts an active run before a combined comment update", async () => {
     const issue = {
       ...makeIssue("todo"),