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>

doc/execution-semantics.md

@@ -183,6 +183,16 @@ A healthy dispatch state means at least one of these is true:
 
 An assigned `todo` issue is stalled when dispatch was interrupted, no wake remains queued or running, and no recovery path has been opened.
 
+### Agent-assigned `backlog`
+
+This is parked state, not dispatch state.
+
+Assigning an issue normally implies executable intent. When create APIs receive an assignee and no explicit status, Paperclip defaults the issue to `todo` so the assignee has a wake path instead of silently inheriting the unassigned `backlog` default.
+
+An explicit assigned `backlog` issue remains valid when the creator is deliberately parking the work. It must not wake the assignee just because it has an assignee. Paperclip should make that choice visible in activity and UI so operators can distinguish intentional parking from a missed handoff.
+
+An assigned `backlog` issue becomes a liveness problem when another issue is blocked on it and there is no explicit waiting path such as a human owner, active run, queued wake, pending interaction or approval, monitor, or open recovery issue. In that case the blocked parent should surface "blocked by parked work" rather than treating the dependency chain as healthy.
+
 ### Agent-assigned `in_progress`
 
 This is active-work state.