feat: add citation/reference support to deep research reports (#1143)

* feat: add citation/reference support to deep research reports (#1141)

- Enhance lead agent system prompt with mandatory citation requirements
  after web_search/web_fetch tool usage
- Add citation examples and best practices to GitHub Deep Research skill
- Add citation hints to report template (Executive Summary, Key Analysis)
- Style regular markdown links in frontend for visual distinction
  (color, underline, hover effect)
- Fix TitleMiddleware being registered when title generation is disabled

* fix: address PR review comments

- Revert TitleMiddleware conditional registration (agent.py) to avoid
  sync/async incompatibility with DeerFlowClient
- Fix markdown link rendering: merge classNames instead of overwriting,
  only set target=_blank for external http(s) URLs
- Remove unrelated package.json/pnpm-lock.yaml changes

* fix: use plain markdown links in Sources section for cleaner rendering

Inline citations in report body use [citation:Title](URL) for pill/badge style.
Sources section uses plain [Title](URL) for simple underlined link style.

* fix(frontend): render plain links as underlined text in artifact markdown

Only links with citation: prefix render as Badge pills.
Regular links in Sources section now render as underlined text links.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/components/workspace/artifacts/artifact-file-detail.tsx

@@ -38,7 +38,7 @@ import { checkCodeFile, getFileName } from "@/core/utils/files";
 import { env } from "@/env";
 import { cn } from "@/lib/utils";
 
-import { CitationLink } from "../citations/citation-link";
+import { ArtifactLink } from "../citations/artifact-link";
 import { useThread } from "../messages/context";
 import { Tooltip } from "../tooltip";
 
@@ -274,7 +274,7 @@ export function ArtifactFilePreview({
         <Streamdown
           className="size-full"
           {...streamdownPlugins}
-          components={{ a: CitationLink }}
+          components={{ a: ArtifactLink }}
         >
           {content ?? ""}
         </Streamdown>