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>

maps.key.props.example

@@ -0,0 +1,13 @@
+<!--
+  SEC-1: local Google Maps key injection for Android builds.
+
+  Copy this file to `maps.key.props` (same directory) and paste your restricted key.
+  `maps.key.props` is gitignored — never commit a real key. CI supplies the key instead
+  via `-p:MapsApiKey=$SECRET` or the MAPS_API_KEY environment variable, so this file is
+  only for local development.
+-->
+<Project>
+  <PropertyGroup>
+    <MapsApiKey>YOUR_GOOGLE_MAPS_ANDROID_API_KEY</MapsApiKey>
+  </PropertyGroup>
+</Project>