fix(vscode): remove duplicate Agent Manager empty state

[marius-kilocode]

When the last managed worktree is removed, selection moves to an empty local context. Agent Manager currently renders both the legacy empty-state block and the newer detail-stack empty state, producing duplicate empty-state CTAs.

This removes the obsolete top-level render so the detail stack is the sole owner of session-less selected contexts while preserving setup-terminal layout behavior.

Before and after:

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[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

lgtm — the removed top-level empty state is fully covered by the detail-stack empty state: contextEmpty() can only be true when a selection exists, which guarantees showDetailStack() renders the remaining panel. All CSS classes, the noSessions i18n key, and the contextEmpty memo remain in use, and the changeset is present and user-facing.

Files Reviewed (2 files)

• .changeset/single-agent-manager-empty-state.md
• packages/kilo-vscode/webview-ui/agent-manager/AgentManagerApp.tsx

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_{Reviewed by kimi-k3 · Input: 59.6K · Output: 4.9K · Cached: 218.5K}

_{Review guidance: REVIEW.md from base branch main}