contracts: extend workstation examples for Mac-on-Linux polish signals

[Copilot]

The workstation contract examples lacked representation of Mac-on-Linux polish surfaces shipped in SociOS-Linux/source-os (screenshot helper, Quick Look/Sushi, sidebar bookmarks, appearance defaults, keyboard remap policy, aggregate polish validation).

Schema extensions (additive, non-breaking)

WorkstationProfile

• macOnLinuxPolish — optional object capturing active polish signals: status (enum: active | planned | non-goal), screenshotHelperCommand, quickLookPackages, keymapPolicyRef
• validation.polishValidationCommand — aggregate polish validation entrypoint

DesktopProfile

• appearance — bounded appearance defaults: colorScheme, accentColor, fontScaling
• sidebarBookmarks — Finder-style sidebar bookmark list (label + path)

Updated examples

workstationprofile.json:

"macOnLinuxPolish": {
  "status": "active",
  "screenshotHelperCommand": "sourceos-screenshot",
  "quickLookPackages": ["gnome-sushi"],
  "keymapPolicyRef": "urn:srcos:keymap-profile:mac-linux-primary"
},
"validation": {
  "polishValidationCommand": "sourceos polish validate --json"
}

desktopprofile.json:

"appearance": { "colorScheme": "dark", "fontScaling": 1.0 },
"sidebarBookmarks": [
  { "label": "Home", "path": "~/" },
  { "label": "Code", "path": "~/code" }
]

Docs

docs/workstation-contract-family.md gains a Mac-on-Linux polish signals section that:

• Tables active signals against their contract fields
• Explicitly lists future/non-goal signals (deep MIME registration, per-app appearance, full macOS parity)
• States implementation authority remains in SociOS-Linux/source-os