Add workspace description text color setting

[teamleaderleo]

Summary

Closes #13438.

• add sidebar.workspaceDescriptionColor as a nullable #RRGGBB override; unset/null keeps the existing theme-derived description color
• wire the value through cmux.json parsing, the settings catalog/snapshot, Sidebar settings UI + search, schema/template/generated validator, and settings docs
• apply the override in both SwiftUI and AppKit sidebar rows, including Markdown link text, while leaving notification preview styling unchanged

Example:

{
  "sidebar": {
    "workspaceDescriptionColor": "#A6E3A1"
  }
}

Regression coverage

The PR keeps the repository's red/green layout as two commits:

1. test: cover workspace description sidebar color
2. feat: add workspace description sidebar color

Coverage includes:

• AppKit row behavior for active and inactive workspaces
• real cmux.json parsing and hex normalization
• SwiftUI Markdown/link coloring
• settings-search row anchoring and schema/supported-path consistency through existing CI guards

The test-only regression was verified red before the implementation: both active/inactive cases rendered the prior secondary color instead of #A6E3A1.

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[teamleaderleo]

#13597 already contains the full implementation for #13438 across settings, schema/config, SwiftUI + AppKit rendering, markdown link color, and regression coverage. This branch currently contains only the test-first AppKit regression.

Please consolidate rather than continue a second implementation track: ensure any useful red-proof assertion here is present in #13597, then treat #13597 as the canonical PR for the issue and close/supersede this branch.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[teamleaderleo]

Superseded by #13597. The canonical PR already carries the AppKit regression for the workspace-description color override, plus settings-file parsing and SwiftUI/markdown coverage, so there is no useful coverage left unique to this branch.