docs(macos): installing the unsigned build on macOS 15+ and its limitations #1594

Problem

docs/macos-signing.md and the README do not describe running an unsigned macOS build, and the existing text is outdated or inconsistent.

• Only "right-click → Open" is documented. On macOS 15+ that no longer bypasses Gatekeeper; users must use System Settings → Privacy & Security → Open Anyway, or xattr -dr com.apple.quarantine.
• Running straight from ~/Downloads triggers App Translocation (read-only random path), which breaks in-place updates (fix(updater): macOS in-place update can quit the app and leave a broken bundle #1587).
• Step 4 of the document says signing applies Release.entitlements, the last bullet says ReleaseSigned.entitlements.
• No note that unsigned builds are sandboxed (docs(macos): sandboxed vs non-sandboxed builds and the minimum macOS version #1591) and what stops working.

Tasks

• README: "Installing the unsigned macOS build" (steps for macOS 14 and 15+, move to /Applications first, xattr alternative).
• docs/macos-signing.md: fix the entitlements contradiction; add a limitations table for unsigned/sandboxed builds.
• Cross-link the Keychain and updater issues.