feat: add community infrastructure and update server setup

- releases/manifest.json: Seed release manifest for update server
- update.rs: Make UPDATE_MANIFEST_URL configurable via ARCHIPELAGO_UPDATE_URL env var
- CONTRIBUTING.md: Comprehensive contribution guidelines covering code style,
  PR process, testing, security disclosure, and app submission
- .github/ISSUE_TEMPLATE/: Bug report, feature request, and app submission
  issue templates with structured forms
- .github/pull_request_template.md: PR template with checklist

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

releases/manifest.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.1.0",
+  "release_date": "2026-03-11",
+  "changelog": [
+    "Initial release with core node management",
+    "Bitcoin, Lightning, and Fedimint app support",
+    "Web5 identity and credentials management",
+    "Auto-update system with rollback support"
+  ],
+  "components": []
+}