[Feat] Extend `@` context picker to attach files, memories, bookmarks & more

[cenktekin]

What problem does this solve? (describe the problem you're experiencing)

• Currently, pressing @ in Chat (Assistant mode) only allows attaching tabs. But when working on a task, I often need to reference files from the workspace, past memories/conversations, bookmarks, or scheduled task outputs — not just open tabs.
• The @ shortcut feels underutilized — it could be a powerful context picker like IDE-style @ menus (VS Code, Cursor, etc.) that can attach any type of context.

How are you working around this today? (your current solution or workaround)

• Manually copy-pasting file contents into the chat
• Leaving tabs open just to attach them via @
• Using memory search as a separate step before asking questions
• No good way to attach bookmarks or scheduled task outputs as context

What's your proposed solution? (how should BrowserOS address this?)

• Extend the @ picker to support multiple context types beyond tabs:
  • Anton Tukhfatullin (@tab) (existing) — attach open browser tabs
  • Fine Free File Command (@file) (new) — browse and attach files from the workspace/filesystem
  • memory (new) — search and attach relevant past memories/daily notes
  • Ngo Xuan Huy (@bookmark) (future) — attach bookmarked pages
  • task (future) — attach scheduled task outputs
• The picker should filter results as you type (fuzzy search across all context types
• Architecture: build a shared context picker component + indexing/search module. Each new source type just needs an adapter.
• V1 = Tab (existing) + File + Memory. V2+ = Bookmark, Task output, Screenshot, etc.

Additional context (optional - add screenshots, examples, or other helpful details)

• This mirrors the IDE pattern (VS Code/Cursor @ menus) — low learning curve for dev users
• A single @ press could show all available types, then filter as you type: e.g. typing "do" could match files like "docker-compose.yml", memories about "docker setup", and tabs with "docker" in the title
• Related: this is a natural companion to the scheduled task grouping feature ([Feat] Group scheduled task results by task name with collapse/expand #950) — task outputs could also be attached as context

---

Thanks for sharing your idea! We love hearing how we can make BrowserOS better. ✨