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/components/IssueChatThread.tsx

@@ -133,6 +133,7 @@ import { Popover, PopoverContent, PopoverTrigger } from "@/components/ui/popover
 import { Textarea } from "@/components/ui/textarea";
 import { AlertTriangle, ArrowRight, Brain, Check, ChevronDown, ClipboardList, Copy, Hammer, Loader2, MoreHorizontal, Paperclip, PauseCircle, Search, Square, ThumbsDown, ThumbsUp } from "lucide-react";
 import { IssueBlockedNotice } from "./IssueBlockedNotice";
+import { IssueAssignedBacklogNotice } from "./IssueAssignedBacklogNotice";
 
 interface IssueChatMessageContext {
   feedbackDataSharingPreference: FeedbackDataSharingPreference;
@@ -296,6 +297,9 @@ interface IssueChatThreadProps {
   blockedBy?: IssueRelationIssueSummary[];
   blockerAttention?: IssueBlockerAttention | null;
   successfulRunHandoff?: SuccessfulRunHandoffState | null;
+  assigneeUserId?: string | null;
+  onResumeFromBacklog?: () => Promise<void> | void;
+  resumeFromBacklogPending?: boolean;
   companyId?: string | null;
   projectId?: string | null;
   issueStatus?: string;
@@ -3650,6 +3654,9 @@ export function IssueChatThread({
   issueWorkMode,
   onWorkModeChange,
   onRefreshLatestComments,
+  assigneeUserId = null,
+  onResumeFromBacklog,
+  resumeFromBacklogPending = false,
 }: IssueChatThreadProps) {
   const location = useLocation();
   const lastScrolledHashRef = useRef<string | null>(null);
@@ -4230,6 +4237,13 @@ export function IssueChatThread({
             )}
               {showComposer ? (
                 <div data-testid="issue-chat-thread-notices" className="space-y-2">
+                  <IssueAssignedBacklogNotice
+                    issueStatus={issueStatus ?? ""}
+                    assigneeAgent={assignedAgent}
+                    assigneeUserId={assigneeUserId}
+                    onResume={onResumeFromBacklog}
+                    resuming={resumeFromBacklogPending}
+                  />
                   <IssueBlockedNotice
                     issueStatus={issueStatus}
                     blockers={unresolvedBlockers}