docs: add workstation-v0 macOS shortcut map contract

[Copilot]

The workstation-v0 Mac-on-Linux lane lacked a single authoritative document distinguishing currently-enforced GNOME shortcuts from planned macOS-parity bindings, making it unclear what is active vs. aspirational.

Changes

docs/workstation/shortcut-map.md (new)

Bounded shortcut contract with two explicit sections:

• Active bindings — 7 shortcuts currently enforced by palette-hotkey.sh and mac-defaults.sh, tabulated with GNOME slot and owning helper:

  Shortcut	Action	Enforced by
  Super+Space	SourceOS palette	palette-hotkey.sh
  Super+E	Files (Nautilus)	mac-defaults.sh
  Super+Return	Terminal	mac-defaults.sh
  Super+Shift+3–6	Screenshots / folder	mac-defaults.sh

• Proposed / future bindings (non-active) — standard macOS Cmd-key equivalents (copy/paste/cut/undo/close/quit/app-switch/window-switch) explicitly marked future with no backing config entries yet.

• Validation commands and non-goals section matching issue boundaries.

tests/workstation-shortcut-map-contract.nix (new)

Nix smoke test (same pattern as workstation-mac-defaults-contract.nix) asserting the document exists, all 7 active shortcuts are present, active/future distinction is captured, and all three helper scripts are referenced.

docs/workstation/README.md

Cross-reference to the new shortcut-map document added to "Related docs".

Validation

# Both commands from the issue pass
test -s docs/workstation/shortcut-map.md
grep -R "Super+Shift+3" docs/workstation profiles/linux-dev/workstation-v0/gnome

[mdheller]

Replay review:

This PR is useful but non-mergeable against current main; do not merge stale history.

Preserve the useful content:

• a bounded workstation-v0 shortcut contract;
• explicit separation of active GNOME bindings from proposed/future macOS-style bindings;
• validation coverage for the active bindings and helper references.

Recommended action: replay this document/test into the fresh workstation acceptance cleanup branch, preferably alongside the canonical acceptance matrix from #129. After that replacement PR is open, close this PR as superseded.