@agent autocomplete for agent teams teammate selection

Feature Request

Problem

When using agent teams with multiple custom agents defined in ~/.claude/agents/, there's no way to discover or autocomplete agent names when typing @. Users must memorize all agent names to invoke them, which doesn't scale as the roster grows.

Current Behavior

• Typing @ in the CLI does nothing — no suggestions, no dropdown, no tab-completion
• The only way to see available agents is /agents command (separate action)
• Shift+Up/Down only works after teammates are already running
• No discoverability for new users or large agent rosters

Proposed Solution

When typing @ in the Claude Code input, show an autocomplete dropdown/list of:

1. Custom agents from ~/.claude/agents/ (name + description from frontmatter)
2. Active teammates currently running in the session
3. Built-in agent types (if applicable)

Similar to how GitHub, Slack, and Discord show @mention suggestions as you type.

Example UX

> @
┌──────────────────────────────────────────────┐
│ @orchestrator    Brain — plans & delegates   │
│ @kimi-coder      Primary coding agent        │
│ @code-reviewer   Reviews code for bugs       │
│ @swift-reviewer  iOS/Swift code review       │
│ @debugger        Diagnoses errors            │
│ @test-runner     Runs & fixes tests          │
└──────────────────────────────────────────────┘

Typing @co would filter to @code-reviewer.

Context

• Agent teams is behind CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
• teammateMode: "tmux" enables split-pane swarm view
• Custom agents are defined as markdown files with YAML frontmatter in ~/.claude/agents/
• The name and description fields from frontmatter provide all the data needed for autocomplete
• This becomes critical as users build larger agent rosters (6+ agents)

Additional Context

Using the tmux swarm view with multiple specialized agents (orchestrator, coders, reviewers). The split-pane UI is excellent for observing agents work in parallel, but the lack of @-autocomplete makes it hard to quickly message or invoke the right teammate without memorizing names.