Tag workspace members from chat to reference, assign, or pull context

[garden-ai-dev]

Problem

Users cannot tag or mention other workspace members directly from chat. This makes it hard to reference someone's earlier work, assign them a task, or pull context from something they created.

Proposed solution

Add member mentions (@name) to chat. Typing @ should show a list of workspace members and let the user pick one. The mention should be actionable: clicking it opens the member's profile/context panel.

Use cases

1. Reference: "As @alice mentioned earlier, we should reuse the schema from her doc."
2. Assign / hand off: "@bob can you pick up the next part?" with optional one-click convert-to-issue assigned to them.
3. Pull context: Mentioning a member surfaces recent issues, docs, or contexts they created so the user can pull from them without losing the thread.

Acceptance criteria

• Typing @ in chat opens a member picker scoped to the current workspace.
• Selected mentions render as clickable chips.
• Mentioned members get a notification or a clear unread indicator.
• Mentions can be used to assign issues without leaving chat.
• Clicking a mention shows the member's recent created contexts (issues, docs, comments).
• Mentions are persisted and searchable in chat history.