Refactor to unified picker showing both agents and files

tasks/done/024-agent-attachments.md

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+---
+title: Agent Attachments
+description: Allow @agent mentions for multi-agent conversations.
+---
+
+Implement Agent Attachments
+
+---
+
+### @ Agent Autocomplete
+
+When a user types `@` followed by an agent name, display an autocomplete list of available agents.
+
+Filter agent suggestions as the user types.
+
+---
+
+### Attach Agents
+
+Enable users to select an agent from the autocomplete list to attach to their prompt.
+
+Display attached agents as interactive chips.
+
+---
+
+### Acceptance Criteria
+
+- Typing `@` followed by a partial agent name displays matching agent suggestions.
+- Selecting an agent creates an attachment chip.
+- Attached agents are included in the prompt submission.
+- Agent chips can be removed.