Add first-class issue references (#4214)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - Operators and agents coordinate through company-scoped issues,
comments, documents, and task relationships.
> - Issue text can mention other tickets, but those references were
previously plain markdown/text without durable relationship data.
> - That made it harder to understand related work, surface backlinks,
and keep cross-ticket context visible in the board.
> - This pull request adds first-class issue reference extraction,
storage, API responses, and UI surfaces.
> - The benefit is that issue references become queryable, navigable,
and visible without relying on ad hoc text scanning.

## What Changed

- Added shared issue-reference parsing utilities and exported
reference-related types/constants.
- Added an `issue_reference_mentions` table, idempotent migration DDL,
schema exports, and database documentation.
- Added server-side issue reference services, route integration,
activity summaries, and a backfill command for existing issue content.
- Added UI reference pills, related-work panels, markdown/editor mention
handling, and issue detail/property rendering updates.
- Added focused shared, server, and UI tests for parsing, persistence,
display, and related-work behavior.
- Rebased `PAP-735-first-class-task-references` cleanly onto
`public-gh/master`; no `pnpm-lock.yaml` changes are included.

## Verification

- `pnpm -r typecheck`
- `pnpm test:run packages/shared/src/issue-references.test.ts
server/src/__tests__/issue-references-service.test.ts
ui/src/components/IssueRelatedWorkPanel.test.tsx
ui/src/components/IssueProperties.test.tsx
ui/src/components/MarkdownBody.test.tsx`

## Risks

- Medium risk because this adds a new issue-reference persistence path
that touches shared parsing, database schema, server routes, and UI
rendering.
- Migration risk is mitigated by `CREATE TABLE IF NOT EXISTS`, guarded
foreign-key creation, and `CREATE INDEX IF NOT EXISTS` statements so
users who have applied an older local version of the numbered migration
can re-run safely.
- UI risk is limited by focused component coverage, but reviewers should
still manually inspect issue detail pages containing ticket references
before merge.

> 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, GPT-5-based coding agent, tool-using shell workflow with
repository inspection, git rebase/push, typecheck, and focused Vitest
verification.

## 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: dotta <dotta@example.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/ActivityRow.tsx

@@ -1,5 +1,6 @@
 import { Link } from "@/lib/router";
 import { Identity } from "./Identity";
+import { IssueReferenceActivitySummary } from "./IssueReferenceActivitySummary";
 import { timeAgo } from "../lib/timeAgo";
 import { cn } from "../lib/utils";
 import { formatActivityVerb } from "../lib/activity-format";
@@ -50,19 +51,22 @@ export function ActivityRow({ event, agentMap, userProfileMap, entityNameMap, en
   const actorAvatarUrl = userProfile?.image ?? null;
 
   const inner = (
-    <div className="flex gap-3">
-      <p className="flex-1 min-w-0 truncate">
-        <Identity
-          name={actorName}
-          avatarUrl={actorAvatarUrl}
-          size="xs"
-          className="align-baseline"
-        />
-        <span className="text-muted-foreground ml-1">{verb} </span>
-        {name && <span className="font-medium">{name}</span>}
-        {entityTitle && <span className="text-muted-foreground ml-1">— {entityTitle}</span>}
-      </p>
-      <span className="text-xs text-muted-foreground shrink-0 pt-0.5">{timeAgo(event.createdAt)}</span>
+    <div className="space-y-2">
+      <div className="flex gap-3">
+        <p className="flex-1 min-w-0 truncate">
+          <Identity
+            name={actorName}
+            avatarUrl={actorAvatarUrl}
+            size="xs"
+            className="align-baseline"
+          />
+          <span className="text-muted-foreground ml-1">{verb} </span>
+          {name && <span className="font-medium">{name}</span>}
+          {entityTitle && <span className="text-muted-foreground ml-1">— {entityTitle}</span>}
+        </p>
+        <span className="text-xs text-muted-foreground shrink-0 pt-0.5">{timeAgo(event.createdAt)}</span>
+      </div>
+      <IssueReferenceActivitySummary event={event} />
     </div>
   );