(feat) Mention artifact files in the chat composer with @

[ashrafchowdury]

When I want the agent to work on a file it already produced, I have no way to point at it. I either describe the path in prose and hope the agent resolves the right file, or I re-upload a copy through the paperclip — even though the session drive already holds the artifact. Naming a file should be as quick as naming a command.

Typing @ in the composer should open the same palette that / opens today, listing the session's artifacts:

• Search across the drive. Typing after the @ filters files and folders by name everywhere in the drive, not just the folder currently in view.
• Drill into folders. Selecting a folder enters it and scopes further typing to its contents, with a way back out to the parent.
• Pick a folder or a file. Selecting a file references that file; selecting a folder references the whole directory, so the agent gets everything under it as context.
• Full keyboard navigation. Arrows move, Enter selects, Tab drills into a folder, Escape steps back a level and then closes. The whole flow works without the mouse, the way the / palette does.
• Open only at a word boundary, so an email address or a handle typed mid-sentence never triggers the menu.
• Insert a real reference, not literal text — the mention resolves to a drive path the agent receives, and it renders in the message as a chip a reader can recognize.

What exists today to build on:

• The / palette: web/packages/agenta-ui/src/RichChatInput/plugins/SlashCommandPlugin.tsx and web/packages/agenta-ui/src/RichChatInput/assets/slashCommands.ts, wired through web/packages/agenta-chat/src/components/ChatComposer.tsx and web/oss/src/components/AgentChatSlice/hooks/useChatSlashCommands.tsx.
• The drive that holds the artifacts: web/packages/agenta-entity-ui/src/drive/useDriveTreeData.ts and web/oss/src/components/Drives/chatFileRefs.tsx, which already turns a file reference into a link in a rendered message.

The composer is shared, so the mention menu should land for every surface that mounts ChatComposer.

[linear-code]

AGE-4160