fix(vscode): refine auto-merge split button

[marius-kilocode]

What Problem This Solves

The Agent Manager auto-merge split button had an uneven action layout and a visually disconnected dropdown chevron. The chevron could also render a dark fill artifact instead of a clean stroke.

Why This Change Was Made

The primary merge action now fills the available footer column and centers its label. The dropdown trigger uses a consistent compact hit area, muted theme-aware colors, a seamless blue surface, and a dedicated right-segment hover state. The chevron is explicitly stroke-only, rotates when expanded, and respects reduced-motion preferences.

User Impact

Auto-merge controls now match the existing split-button interaction pattern while retaining the blue primary-action treatment. Hovering the right segment clearly identifies the dropdown without introducing a hard divider or an overly bright icon.

Evidence

Normal state:

Right-segment hover state:

Validation completed with bun run compile, the focused Agent Manager architecture test suite, and isolated VS Code visual checks for normal and right-segment hover states.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• .changeset/refine-pr-merge-split-button.md
• packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css
• packages/kilo-vscode/webview-ui/agent-manager/pr/pr-panel.css

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_{Reviewed by grok-4.6 · Input: 85.7K · Output: 14.7K · Cached: 1M}

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