Command center: dim Agents pill when inactive; soften quick input tabs

[srikanthananthula (srikanthananthula63053)]

Addresses part of #331016.

What

• The Agents window's command-center pill never dimmed when the window
  lost focus, unlike the classic workbench command-center pill. Adds
  the missing inactive-state rule.
• The experimental "Agent quick input" tab bar (Sessions/Commands/Files)
  read as heavier than the plain single-column Command Palette next to
  it. Tightens its padding/gap and swaps the active tab's solid
  quickInputList-focus* fill for a subtle focusBorder tint.

What this intentionally does NOT change

The two pills' at-rest background (solid vs. transparent) was left
alone. styleOverrides/commandCenter.css documents the Agents pill's
transparent-at-rest look as the reference the classic pill is already
being migrated toward under workbench.experimental.modernUI — matching
them by making the Agents pill solid would go the wrong direction and
stale that doc comment.

Known, disclosed inconsistency: runScriptAction.css's radio-tab
.active state still uses the solid quickInputList-focus* fill this
PR moves away from in unifiedQuickAccess.css. No documented contract
ties the two together (verified against the base Radio component's
own defaults, which neither file uses), but flagging for reviewer
awareness in case a follow-up should unify them.

Test plan

• npm run stylelint on both files: 0 errors (15 pre-existing/advisory
  design-token suggestions on untouched or convention-matched lines).
• npm run typecheck-client: no new errors (2 pre-existing errors in
  unrelated agentHost test files, confirmed present on bare
  origin/main before this change).
• Manually verified live in Code OSS built from source: confirmed via
  getComputedStyle that all changed properties resolve to the
  intended values, and that Sessions/Commands tab-switching still
  functions correctly after the styling change.

[Copilot]

Pull request overview

Dims the Agents command-center pill in inactive windows and softens experimental quick-input tabs.

Changes:

• Adds inactive title-bar styling.
• Reduces tab spacing and visual emphasis.
• Replaces the active tab’s solid fill with a subtle tint.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
unifiedQuickAccess.css	Refines tab spacing, weight, and active styling.
sessionsTitleBarWidget.css	Adds inactive command-center colors.

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