Add ordered sub-issue navigation (#5938)

## Thinking Path

> - Paperclip orchestrates AI-agent companies through company-scoped
issues, comments, and execution context.
> - The issue detail page is the board surface where operators and
agents inspect a task in its parent/child workflow.
> - Ordered sub-issues need a low-friction way to move through work
without returning to the parent list after every issue.
> - Existing issue detail navigation only covered sibling transitions
and did not continue into a parent issue's first ordered child.
> - This pull request adds ordered previous/next navigation for issue
detail views and extends it to continue from a parent or last sibling
into the first direct child.
> - The benefit is a smoother review/execution path through hierarchical
work while preserving hidden issue filtering and dependency-aware
ordering.

## What Changed

- Added `IssueSiblingNavigation` and route-state handling so issue
detail footers can link to previous/next ordered issues.
- Extended sub-issue ordering helpers to build navigation from siblings
plus direct children, including root-parent and
last-sibling-to-first-child cases.
- Added page, component, and library tests for ordered sibling
navigation, child fallback navigation, hidden issues, and link
rendering.
- Fixed the quicklook blur/click race Greptile found by deferring close
until after portaled link clicks can complete, with a regression test.
- Polished the navigation landmark label so it remains accurate when the
next target is a direct child rather than a sibling.

## Verification

- `pnpm exec vitest run src/components/IssueLinkQuicklook.test.tsx
src/lib/issue-detail-subissues.test.ts
src/components/IssueSiblingNavigation.test.tsx
src/pages/IssueDetail.test.tsx --config vitest.config.ts` from `ui/` -
31 tests passed.
- `pnpm --filter @paperclipai/ui typecheck` - passed.
- `git diff --check` - passed.
- GitHub PR checks on latest head `34046be2` - passed: Greptile Review,
verify, e2e, Canary Dry Run, policy, Snyk, and serialized server shards.
- Screenshots: not captured in this heartbeat; this PR is a draft and
the changed states are covered by focused component/page tests.

## Risks

- Low risk; this is a UI navigation addition with no database or API
contract changes.
- The main behavioral risk is navigation ordering drift if
`workflowSort` expectations change later.
- The IssueDetail navigation now waits for child issue loading, which
avoids stale child fallback links but can delay footer navigation
briefly while data loads.

> 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, GPT-5 coding agent with repository tool use and shell
execution.

## 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
- [ ] 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

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueChatThread.tsx

@@ -342,6 +342,7 @@ interface IssueChatThreadProps {
   showComposer?: boolean;
   showJumpToLatest?: boolean;
   emptyMessage?: string;
+  footer?: ReactNode;
   variant?: "full" | "embedded";
   enableLiveTranscriptPolling?: boolean;
   transcriptsByRunId?: ReadonlyMap<string, readonly IssueChatTranscriptEntry[]>;
@@ -3650,6 +3651,7 @@ export function IssueChatThread({
   showComposer = true,
   showJumpToLatest,
   emptyMessage,
+  footer,
   variant = "full",
   enableLiveTranscriptPolling = true,
   transcriptsByRunId,
@@ -4310,6 +4312,7 @@ export function IssueChatThread({
                   <IssueAssigneePausedNotice agent={assignedAgent} />
                 </div>
               ) : null}
+              {footer ? <div data-testid="issue-chat-thread-footer">{footer}</div> : null}
               <div ref={bottomAnchorRef} />
               {showComposer ? (
                 <div