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>

packages/db/src/migrations/0060_orange_annihilus.sql

@@ -0,0 +1,50 @@
+CREATE TABLE IF NOT EXISTS "issue_reference_mentions" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"company_id" uuid NOT NULL,
+	"source_issue_id" uuid NOT NULL,
+	"target_issue_id" uuid NOT NULL,
+	"source_kind" text NOT NULL,
+	"source_record_id" uuid,
+	"document_key" text,
+	"matched_text" text,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+DO $$ BEGIN
+ IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'issue_reference_mentions_company_id_companies_id_fk') THEN
+  ALTER TABLE "issue_reference_mentions" ADD CONSTRAINT "issue_reference_mentions_company_id_companies_id_fk" FOREIGN KEY ("company_id") REFERENCES "public"."companies"("id") ON DELETE no action ON UPDATE no action;
+ END IF;
+END $$;
+--> statement-breakpoint
+DO $$ BEGIN
+ IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'issue_reference_mentions_source_issue_id_issues_id_fk') THEN
+  ALTER TABLE "issue_reference_mentions" ADD CONSTRAINT "issue_reference_mentions_source_issue_id_issues_id_fk" FOREIGN KEY ("source_issue_id") REFERENCES "public"."issues"("id") ON DELETE cascade ON UPDATE no action;
+ END IF;
+END $$;
+--> statement-breakpoint
+DO $$ BEGIN
+ IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'issue_reference_mentions_target_issue_id_issues_id_fk') THEN
+  ALTER TABLE "issue_reference_mentions" ADD CONSTRAINT "issue_reference_mentions_target_issue_id_issues_id_fk" FOREIGN KEY ("target_issue_id") REFERENCES "public"."issues"("id") ON DELETE cascade ON UPDATE no action;
+ END IF;
+END $$;--> statement-breakpoint
+CREATE INDEX IF NOT EXISTS "issue_reference_mentions_company_source_issue_idx" ON "issue_reference_mentions" USING btree ("company_id","source_issue_id");--> statement-breakpoint
+CREATE INDEX IF NOT EXISTS "issue_reference_mentions_company_target_issue_idx" ON "issue_reference_mentions" USING btree ("company_id","target_issue_id");--> statement-breakpoint
+CREATE INDEX IF NOT EXISTS "issue_reference_mentions_company_issue_pair_idx" ON "issue_reference_mentions" USING btree ("company_id","source_issue_id","target_issue_id");--> statement-breakpoint
+DELETE FROM "issue_reference_mentions"
+WHERE "id" IN (
+	SELECT "id"
+	FROM (
+		SELECT
+			"id",
+			row_number() OVER (
+				PARTITION BY "company_id", "source_issue_id", "target_issue_id", "source_kind", "source_record_id"
+				ORDER BY "created_at", "id"
+			) AS "row_number"
+		FROM "issue_reference_mentions"
+	) AS "duplicates"
+	WHERE "duplicates"."row_number" > 1
+);--> statement-breakpoint
+DROP INDEX IF EXISTS "issue_reference_mentions_company_source_mention_uq";--> statement-breakpoint
+CREATE UNIQUE INDEX IF NOT EXISTS "issue_reference_mentions_company_source_mention_record_uq" ON "issue_reference_mentions" USING btree ("company_id","source_issue_id","target_issue_id","source_kind","source_record_id") WHERE "source_record_id" IS NOT NULL;--> statement-breakpoint
+CREATE UNIQUE INDEX IF NOT EXISTS "issue_reference_mentions_company_source_mention_null_record_uq" ON "issue_reference_mentions" USING btree ("company_id","source_issue_id","target_issue_id","source_kind") WHERE "source_record_id" IS NULL;

packages/db/src/migrations/meta/_journal.json

@@ -421,6 +421,13 @@
       "when": 1776542246000,
       "tag": "0059_plugin_database_namespaces",
       "breakpoints": true
+    },
+    {
+      "idx": 60,
+      "version": "7",
+      "when": 1776717606743,
+      "tag": "0060_orange_annihilus",
+      "breakpoints": true
     }
   ]
-}
+}