feat: Developer-ID sign + notarize macOS builds

- tauri.conf.json: enable hardenedRuntime + entitlements (keep ad-hoc
  signingIdentity as the dev default; release script overrides it)
- add desktop/src-tauri/entitlements.plist (JIT / unsigned-exec-mem /
  dyld-env-vars for the hardened runtime)
- scripts/release-mac.sh: auto-detect Developer ID identity from the
  keychain, load notarization creds from ~/.config/gridline/notarize.env
  (App Store Connect API key), re-sign bundled pg tools after
  install_name_tool (invalidated signatures would fail notarization),
  and verify codesign + stapled notarization before upload
- README/AGENTS.md: replace the unsigned/xattr instructions with the
  macOS signing & notarization setup guide

desktop/src-tauri/entitlements.plist

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<!-- Required for the WKWebView (JIT-compiled JavaScript). Without this the
+	     app crashes under the hardened runtime. -->
+	<key>com.apple.security.cs.allow-jit</key>
+	<true/>
+	<!-- The WebView allocates executable memory for JIT; allow it. -->
+	<key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+	<true/>
+	<!-- Tauri's dev server / debug builds rely on DYLD_* environment variables. -->
+	<key>com.apple.security.cs.allow-dyld-environment-variables</key>
+	<true/>
+</dict>
+</plist>