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/IssueBlockedNotice.tsx

@@ -1,8 +1,9 @@
 import type { IssueBlockerAttention, IssueRelationIssueSummary, SuccessfulRunHandoffState } from "@paperclipai/shared";
-import { AlertTriangle } from "lucide-react";
+import { AlertTriangle, Flag } from "lucide-react";
 import { Link } from "@/lib/router";
 import { createIssueDetailPath } from "../lib/issueDetailBreadcrumb";
 import { IssueLinkQuicklook } from "./IssueLinkQuicklook";
+import { isAssignedBacklogBlocker } from "../lib/issue-blockers";
 
 export function IssueBlockedNotice({
   issueStatus,
@@ -27,6 +28,24 @@ export function IssueBlockedNotice({
     .filter((blocker, index, all) => all.findIndex((candidate) => candidate.id === blocker.id) === index);
 
   const isStalled = blockerAttention?.state === "stalled";
+  const parkedBlockers = (() => {
+    const seen = new Set<string>();
+    const collected: IssueRelationIssueSummary[] = [];
+    const sources: IssueRelationIssueSummary[] = [...blockers];
+    for (const blocker of blockers) {
+      for (const terminal of blocker.terminalBlockers ?? []) {
+        sources.push(terminal);
+      }
+    }
+    for (const blocker of sources) {
+      if (!isAssignedBacklogBlocker(blocker)) continue;
+      if (seen.has(blocker.id)) continue;
+      seen.add(blocker.id);
+      collected.push(blocker);
+    }
+    return collected;
+  })();
+  const showParkedRow = parkedBlockers.length > 0;
   const stalledLeafIdentifier =
     blockerAttention?.sampleStalledBlockerIdentifier ?? blockerAttention?.sampleBlockerIdentifier ?? null;
   const stalledLeafBlockers = (() => {
@@ -148,6 +167,18 @@ export function IssueBlockedNotice({
                   {terminalBlockers.map(renderBlockerChip)}
                 </div>
               ) : null}
+              {showParkedRow ? (
+                <div
+                  data-testid="issue-blocked-notice-parked-row"
+                  className="flex flex-wrap items-center gap-1.5 pt-0.5"
+                >
+                  <span className="inline-flex items-center gap-1 text-xs font-medium text-amber-800 dark:text-amber-200">
+                    <Flag className="h-3 w-3" aria-hidden />
+                    Blocked by parked work
+                  </span>
+                  {parkedBlockers.map(renderBlockerChip)}
+                </div>
+              ) : null}
             </>
           ) : null}
         </div>