Enhance agent merge widget for accessibility and interaction - #333973

This refreshes the Agent Merge system-initiated chat turn so review feedback, failing checks, and merge status are easier to scan without exposing machine-facing content or persistent secondary chrome.

Changes

• Consolidate review comments, failing checks, conflicts, and branch status into a single severity-led summary.
• Left-align the widget, move the chevron to the trailing edge, use the standard pull request pill, and omit redundant section headings when only one issue type is present.
• Reveal the border, source metadata, and trailing controls together on header hover, keyboard focus, or expansion, without activating from the transparent outer stroke.
• Add a reduced-motion-aware source fade and preserve collapsed widget boundaries in high-contrast themes.
• Add an Agent Message toggle with a stable accessible name, pressed state, keyboard-only focus treatment, and non-sticky pointer focus.
• Attach the full summary hover to the interactive disclosure so truncated status text remains discoverable.
• Add an in-chat fixture using the production chat model, view model, and list widget path, alongside expanded, linked-comment, Markdown, and high-contrast scenarios.
• Update transcript labels, timeline expectations, and focused unit coverage.

Validation

• Focused Agent Merge unit tests passed.
• Targeted ESLint and Stylelint passed.
• VS Code editor diagnostics reported no errors.
• git diff --check passed.
• Component Explorer verified standalone, in-chat, hover, focus, expanded, conditional-section, Agent Message, and dark high-contrast states.

Known unrelated issue: Component Explorer can report duplicate registration of inlineChatWidget.statusPlaceholder during HMR reloads; the Agent Merge fixtures continue to render.