Chat: Restore issue and PR context attachments temporarily

[meganrogge]

Summary

Temporarily restore pasted GitHub issue and pull request URLs as Chat context attachments instead of compact inline references.

Rationale

The compact inline treatment introduced for #334061 should be held until the same presentation is available consistently for all attachment types, as described in #334542.

Scope

• restore automatic issue and pull request context attachments when their URLs are pasted
• restore synchronization of those attachments in the Agents composer
• remove the temporary compact-reference setting and supporting serialization/rendering behavior

Implemented by #334544.

[vs-code-engineering]

Sharing media and context from merged PR #334544 for verification:

Part of #334542

Summary

• revert the compact inline reference treatment for pasted GitHub issue and pull request URLs
• restore pasted issue and pull request URLs as context attachments
• restore automatic GitHub context attachment synchronization in the Agents composer
• remove the temporary chat.pasteGitHubLinksAsReferences setting and compact-reference serialization/rendering support

Validation

• npm run transpile-client
• npm run hygiene
• targeted ESLint on all changed TypeScript files
• ./scripts/test.sh --run src/vs/workbench/contrib/chat/test/browser/attachments/chatVariables.test.ts --run src/vs/workbench/contrib/chat/test/browser/widget/input/editor/chatPasteProviders.test.ts --run src/vs/workbench/contrib/chat/test/browser/widget/chatListRenderer.test.ts --run src/vs/workbench/contrib/chat/test/common/requestParser/chatRequestParser.test.ts --run src/vs/sessions/contrib/chat/test/browser/newChatInputPaste.test.ts --run src/vs/sessions/contrib/chat/test/browser/newChatInput.test.ts (120 passing, 1 pending)

npm run typecheck-client was also attempted, but this checkout is missing Electron and original-fs type dependencies; the resulting errors are repository-wide missing-module errors outside the changed files.

• live Code OSS verification that pasted issue and PR URLs remain in the prompt and create context attachments

Screenshot

Source PR: #334544