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/IssueProperties.tsx

@@ -26,6 +26,7 @@ import { buildExecutionPolicy, stageParticipantValues } from "../lib/issue-execu
 import { StatusIcon } from "./StatusIcon";
 import { PriorityIcon } from "./PriorityIcon";
 import { Identity } from "./Identity";
+import { IssueReferencePill } from "./IssueReferencePill";
 import { formatDate, cn, projectUrl } from "../lib/utils";
 import { timeAgo } from "../lib/timeAgo";
 import { Separator } from "@/components/ui/separator";
@@ -295,6 +296,30 @@ export function IssueProperties({
     if (isMainIssueWorkspace({ issue, project: issueProject })) return null;
     return runningRuntimeServiceWithUrl(issue.currentExecutionWorkspace?.runtimeServices);
   }, [issue, issueProject]);
+  const referencedIssueIdentifiers = issue.referencedIssueIdentifiers ?? [];
+  const relatedTasks = useMemo(() => {
+    const excluded = new Set<string>();
+    const addExcluded = (candidate: { id: string; identifier?: string | null }) => {
+      excluded.add(candidate.id);
+      if (candidate.identifier) excluded.add(candidate.identifier);
+    };
+
+    for (const blocker of issue.blockedBy ?? []) addExcluded(blocker);
+    for (const blocked of issue.blocks ?? []) addExcluded(blocked);
+    for (const child of childIssues) addExcluded(child);
+
+    const referencedIssues = issue.relatedWork?.outbound.map((item) => item.issue) ?? [];
+    if (referencedIssues.length > 0) {
+      return referencedIssues.filter((referenced) => {
+        const label = referenced.identifier ?? referenced.id;
+        return !excluded.has(referenced.id) && !excluded.has(label);
+      });
+    }
+
+    return referencedIssueIdentifiers
+      .filter((identifier) => !excluded.has(identifier))
+      .map((identifier) => ({ id: identifier, identifier, title: identifier }));
+  }, [childIssues, issue.blockedBy, issue.blocks, issue.relatedWork?.outbound, referencedIssueIdentifiers]);
   const projectLink = (id: string | null) => {
     if (!id) return null;
     const project = projects?.find((p) => p.id === id) ?? null;
@@ -1113,12 +1138,22 @@ export function IssueProperties({
                 onClick={onAddSubIssue}
               >
                 <Plus className="h-3 w-3" />
-                Add sub-issue
+              Add sub-issue
               </button>
             ) : null}
           </div>
         </PropertyRow>
 
+        {relatedTasks.length > 0 ? (
+          <PropertyRow label="Related Tasks">
+            <div className="flex flex-wrap gap-1">
+              {relatedTasks.map((related) => (
+                <IssueReferencePill key={related.id} issue={related} />
+              ))}
+            </div>
+          </PropertyRow>
+        ) : null}
+
         <PropertyPicker
           inline={inline}
           label="Reviewers"