[codex] Polish issue composer and long document display (#4420)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Issue comments and documents are the main working surface where
operators and agents collaborate
> - File drops, markdown editing, and long issue descriptions need to
feel predictable because they sit directly in the task execution loop
> - The composer had edge cases around drag targets, attachment
feedback, image drops, and long markdown content crowding the page
> - This pull request polishes the issue composer, hardens markdown
editor regressions, and adds a fold curtain for long issue
descriptions/documents
> - The benefit is a calmer issue detail surface that handles uploads
and long work products without hiding state or breaking layout

## What Changed

- Scoped issue-composer drag/drop behavior so the composer owns file
drops without turning the whole thread into a competing drop target.
- Added clearer attachment upload feedback for non-image files and
image-drop stability coverage.
- Hardened markdown editor and markdown body handling around HTML-like
tag regressions.
- Added `FoldCurtain` and wired it into issue descriptions and issue
documents so long markdown previews can expand/collapse.
- Added Storybook coverage for the fold curtain state.

## Verification

- `pnpm exec vitest run ui/src/components/IssueChatThread.test.tsx
ui/src/components/MarkdownEditor.test.tsx
ui/src/components/MarkdownBody.test.tsx --config ui/vitest.config.ts`
passed: 3 files, 75 tests.
- `git diff --check public-gh/master..pap-2228-editor-composer-polish --
. ':(exclude)ui/storybook-static'` passed.
- Confirmed this PR does not include `pnpm-lock.yaml`.

## Risks

- Low-to-medium risk: this changes user-facing composer/drop behavior
and long markdown display.
- The fold curtain uses DOM measurement and `ResizeObserver`; reviewers
should check browser behavior for very long descriptions and documents.
- No database migrations.

> 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 coding agent based on GPT-5, with shell, git, Paperclip
API, and GitHub CLI tool use in the local Paperclip workspace.

## 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
- [x] 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

Note: screenshots were not newly captured during branch splitting; the
UI states are covered by component tests and a Storybook story.

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/pages/IssueDetail.tsx

@@ -578,7 +578,7 @@ type IssueDetailChatTabProps = {
   ) => Promise<void>;
   onAdd: (body: string, reopen?: boolean, reassignment?: CommentReassignment) => Promise<void>;
   onImageUpload: (file: File) => Promise<string>;
-  onAttachImage: (file: File) => Promise<void>;
+  onAttachImage: (file: File) => Promise<IssueAttachment | void>;
   onInterruptQueued: (runId: string) => Promise<void>;
   onCancelQueued: (commentId: string) => void;
   interruptingQueuedRunId: string | null;
@@ -2543,7 +2543,7 @@ export function IssueDetail() {
     return attachment.contentPath;
   }, [uploadAttachment]);
   const handleCommentAttachImage = useCallback(async (file: File) => {
-    await uploadAttachment.mutateAsync(file);
+    return uploadAttachment.mutateAsync(file);
   }, [uploadAttachment]);
   const handleInterruptQueuedRun = useCallback(async (runId: string) => {
     await interruptQueuedComment.mutateAsync(runId);
@@ -3069,6 +3069,7 @@ export function IssueDetail() {
           className="text-[15px] leading-7 text-foreground"
           placeholder="Add a description..."
           multiline
+          foldable
           mentions={mentionOptions}
           imageUploadHandler={async (file) => {
             const attachment = await uploadAttachment.mutateAsync(file);