Guard assigned backlog liveness (#5428)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The issue graph and liveness recovery system decide whether assigned
work is executable or parked
> - Assigned issues created without an explicit status could silently
land in backlog, making parents look blocked with no productive wake
path
> - The server, shared validators, recovery analysis, and UI all need to
agree on that execution semantic
> - This pull request makes assigned issue creation default to `todo`,
flags assigned backlog blockers, and surfaces the state in the board
> - The benefit is that parked assigned work becomes intentional and
visible instead of creating silent liveness stalls

## What Changed

- Adds contract tests for assigned issue creation defaults.
- Defaults assigned issue creation to `todo` when status is omitted
while preserving explicit `backlog` parking.
- Exposes `resolveCreateIssueStatusDefault` through shared validators.
- Teaches liveness/blocker attention paths to distinguish assigned
backlog blockers.
- Adds UI notices, row/header badges, and issue detail safeguards for
assigned backlog blockers.
- Adds Storybook fixtures and execution-semantics documentation for the
assigned-backlog behavior.

## Verification

- `pnpm run preflight:workspace-links && pnpm exec vitest run
packages/shared/src/validators/issue.test.ts
server/src/__tests__/issue-assigned-backlog-contract-routes.test.ts
server/src/__tests__/issue-blocker-attention.test.ts
server/src/__tests__/issue-liveness.test.ts
server/src/__tests__/heartbeat-issue-liveness-escalation.test.ts
ui/src/components/IssueAssignedBacklogNotice.test.tsx
ui/src/components/IssueRow.test.tsx` — 50 passed, 23 skipped.
- Skipped tests were embedded Postgres suites on this host with the repo
skip message: `Postgres init script exited with code null. Please check
the logs for extra info. The data directory might already exist.`
- Pairwise merge check against the issue-controls PR branch completed
without conflicts via `git merge --no-commit --no-ff` in a temporary
worktree.
- Screenshots for assigned-backlog UI states:
[light](docs/pr-screenshots/pr-5428/assigned-backlog-light.png),
[dark](docs/pr-screenshots/pr-5428/assigned-backlog-dark.png).
- Follow-up checks: `pnpm --filter /ui typecheck`; `pnpm --filter
/mcp-server build`; `pnpm --filter /mcp-server test`; `pnpm exec vitest
run packages/shared/src/validators/issue.test.ts`; focused UI component
tests.
- Remote PR checks on head `6300b3c`: policy, verify, serialized server
shards 1/4-4/4, Canary Dry Run, e2e, Greptile Review, and Snyk all
passed.

## Risks

- Medium: changes status defaulting for assigned issue creation when the
caller omits status. Explicit `backlog` remains supported, and
server/shared tests cover both paths.
- Medium: liveness classification changes can affect blocker attention
labels; focused service and UI tests cover the new assigned-backlog
state.

> 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, GPT-5 model family (`gpt-5`), tool-enabled
Paperclip heartbeat environment. Context window and internal reasoning
mode are not exposed by the runtime.

## 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

---------

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

ui/src/pages/IssueDetail.tsx

@@ -110,6 +110,7 @@ import {
   SUCCESSFUL_RUN_HANDOFF_REQUIRED_ACTION,
   successfulRunHandoffActivityTone,
 } from "../lib/successful-run-handoff";
+import { hasAssignedBacklogBlocker } from "../lib/issue-blockers";
 import {
   Activity as ActivityIcon,
   AlertTriangle,
@@ -120,6 +121,7 @@ import {
   Copy,
   Eye,
   EyeOff,
+  Flag,
   Hexagon,
   ListTree,
   MessageSquare,
@@ -644,6 +646,9 @@ type IssueDetailChatTabProps = {
     answers: AskUserQuestionsAnswer[],
   ) => Promise<void>;
   onCancelInteraction: (interaction: AskUserQuestionsInteraction) => Promise<void>;
+  assigneeUserId: string | null;
+  onResumeFromBacklog?: () => Promise<void> | void;
+  resumeFromBacklogPending?: boolean;
 };
 
 const IssueDetailChatTab = memo(function IssueDetailChatTab({
@@ -694,6 +699,9 @@ const IssueDetailChatTab = memo(function IssueDetailChatTab({
   onRejectInteraction,
   onSubmitInteractionAnswers,
   onCancelInteraction,
+  assigneeUserId,
+  onResumeFromBacklog,
+  resumeFromBacklogPending,
 }: IssueDetailChatTabProps) {
   const { data: activity } = useQuery({
     queryKey: queryKeys.issues.activity(issueId),
@@ -901,6 +909,9 @@ const IssueDetailChatTab = memo(function IssueDetailChatTab({
           : undefined}
         onImageClick={onImageClick}
         onRefreshLatestComments={onRefreshLatestComments}
+        assigneeUserId={assigneeUserId}
+        onResumeFromBacklog={onResumeFromBacklog}
+        resumeFromBacklogPending={resumeFromBacklogPending}
       />
     </div>
   );
@@ -2894,6 +2905,10 @@ export function IssueDetail() {
   const handleCancelInteraction = useCallback(async (interaction: AskUserQuestionsInteraction) => {
     await cancelInteraction.mutateAsync({ interaction });
   }, [cancelInteraction]);
+  const canResumeFromBacklog = issue?.status === "backlog" && Boolean(issue.assigneeAgentId || issue.assigneeUserId);
+  const handleResumeFromBacklog = useCallback(async () => {
+    await updateIssue.mutateAsync({ status: "todo" });
+  }, [updateIssue.mutateAsync]);
 
   const treePreviewAffectedIssues = useMemo(
     () => (treeControlPreview?.issues ?? []).filter((candidate) => !candidate.skipped),
@@ -3240,6 +3255,17 @@ export function IssueDetail() {
             </span>
           ) : null}
 
+          {hasAssignedBacklogBlocker(issue.blockedBy) ? (
+            <span
+              data-testid="issue-detail-parked-blocker"
+              className="inline-flex items-center gap-1 rounded-full border border-amber-500/60 bg-amber-500/15 px-2 py-0.5 text-[10px] font-medium text-amber-700 dark:text-amber-300 shrink-0"
+              title="Blocked by parked work — at least one assigned blocker is in backlog and will not wake its assignee."
+            >
+              <Flag className="h-3 w-3" />
+              Blocked by parked work
+            </span>
+          ) : null}
+
           {issue.projectId ? (
             <Link
               to={`/projects/${issue.projectId}`}
@@ -3805,6 +3831,11 @@ export function IssueDetail() {
               onRejectInteraction={handleRejectInteraction}
               onSubmitInteractionAnswers={handleSubmitInteractionAnswers}
               onCancelInteraction={handleCancelInteraction}
+              assigneeUserId={issue.assigneeUserId ?? null}
+              onResumeFromBacklog={canResumeFromBacklog ? handleResumeFromBacklog : undefined}
+              resumeFromBacklogPending={
+                updateIssue.isPending && updateIssue.variables?.status === "todo"
+              }
             />
           ) : null}
         </TabsContent>