SEC-1: build-inject Google Maps Android key; remove leaked literal

Removes the leaked Google Maps API key literal from README.md and the
live com.google.android.geo.API_KEY in AndroidManifest.xml (which shipped
in every built APK). The manifest value is now the build-time placeholder
${MAPS_API_KEY}, injected via AndroidManifestPlaceholders from the
MapsApiKey MSBuild property, resolved from a CI secret (-p:MapsApiKey=),
the MAPS_API_KEY env var, or a gitignored maps.key.props at the repo root
(maps.key.props.example committed as the template). maps.key.props is
gitignored so a real key is never committed.

No rotated key is included here; the human supplies it via CI secret.
Pairs with the console key rotation to close SEC-1 (decisions.md
2026-08-20; security/sec-1-gmaps-key.md). Git history intentionally not
rewritten (recorded risk-acceptance relies on revocation of the old key).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

FieldLogger/Platforms/Android/AndroidManifest.xml

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 	<application android:allowBackup="true" android:icon="@mipmap/appicon" android:roundIcon="@mipmap/appicon_round" android:supportsRtl="true">
-		<!-- Google Maps API key: replace with your key from https://console.cloud.google.com (Maps SDK for Android) -->
-		<meta-data android:name="com.google.android.geo.API_KEY" android:value="AIzaSyDhH16gF-7UN-CBsTQGfQSHNGjLC6VJ5dI" />
+		<!-- Google Maps API key is injected at build time from the MAPS_API_KEY MSBuild
+		     placeholder (see FieldLogger.csproj / maps.key.props.example). Never commit a key
+		     literal here — SEC-1. -->
+		<meta-data android:name="com.google.android.geo.API_KEY" android:value="${MAPS_API_KEY}" />
 	</application>
 	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
 	<uses-permission android:name="android.permission.INTERNET" />