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/IssueRow.tsx

@@ -1,7 +1,7 @@
 import type { ReactNode } from "react";
 import type { Issue } from "@paperclipai/shared";
 import { Link } from "@/lib/router";
-import { Eye, X } from "lucide-react";
+import { Eye, Flag, X } from "lucide-react";
 import {
   createIssueDetailPath,
   rememberIssueDetailLocationState,
@@ -10,6 +10,7 @@ import {
 import { cn } from "../lib/utils";
 import { StatusIcon } from "./StatusIcon";
 import { productivityReviewTriggerLabel } from "./ProductivityReviewBadge";
+import { hasAssignedBacklogBlocker } from "../lib/issue-blockers";
 
 type UnreadState = "hidden" | "visible" | "fading";
 
@@ -91,6 +92,16 @@ export function IssueRow({
       Planning
     </span>
   ) : null;
+  const parkedBlockerIndicator = hasAssignedBacklogBlocker(issue.blockedBy) ? (
+    <span
+      data-testid="issue-row-parked-blocker"
+      className="ml-1.5 inline-flex shrink-0 items-center gap-0.5 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"
+      title="Blocked by parked work — at least one assigned blocker is in backlog and will not wake its assignee."
+    >
+      <Flag className="h-2.5 w-2.5" aria-hidden />
+      Blocked by parked work
+    </span>
+  ) : null;
 
   return (
     <Link
@@ -113,6 +124,7 @@ export function IssueRow({
         {mobileLeading ?? <StatusIcon status={issue.status} blockerAttention={issue.blockerAttention} className={selectedStatusClass} />}
         {productivityReviewIndicator}
         {planningModeIndicator}
+        {parkedBlockerIndicator}
       </span>
       <span className="flex min-w-0 flex-1 flex-col gap-1 sm:contents">
         <span className={cn("line-clamp-2 text-sm sm:order-2 sm:min-w-0 sm:flex-1 sm:truncate sm:line-clamp-none", titleClassName)}>
@@ -138,6 +150,7 @@ export function IssueRow({
                 {identifier}
               </span>
               {planningModeIndicator}
+              {parkedBlockerIndicator}
             </>
           )}
           {mobileMeta ? (