chore: establish M0 project foundations

Document accepted MVP decisions, add AGPL-3.0-only licensing, scaffold repository boundaries, and configure verified PSR-12 and foundation checks.

Refs #1, #2, #3, #4, #5

src/README.md

@@ -0,0 +1,6 @@
+# PHP application source
+
+Application/domain services and persistence code belong here. Use the future
+`MeshVault` namespace and keep HTTP handlers separate from storage operations.
+M1 issue #7 adds routing, dependency wiring and PSR-4 autoloading; no runtime
+framework or application entry point exists yet.