Bring Modern UI tab design to the Agents window

[Copilot]

Extends the updated Modern UI tab design to the Agents window without changing its default appearance.

• Scoping

  • Gates Agents tab styling on workbench.experimental.modernUI.
  • Uses an Agents-specific root class to avoid activating unrelated workbench style overrides.

• Editor tabs

  • Adds transparent resting states, stronger active and hover fills, overlaid tab actions, and improved selected, dirty, pinned, and decorated states.
  • Preserves Agents-specific tab sizing and add-tab behavior.

• Pane tabs

  • Aligns panel and auxiliary-bar text and icon tabs with the Modern UI active and hover treatments.
  • Adds explicit high-contrast and keyboard-focus indicators.

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