chore: prepare repository for public launch

SECURITY.md

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+# Security Policy
+
+## Reporting vulnerabilities
+
+Please do not open a public issue for a security vulnerability.
+
+Until a dedicated security intake address is published, report privately to the
+project maintainer through the repository owner account or the private contact
+channel listed on the project homepage.
+
+Include:
+
+- affected commit, version, or release;
+- affected component;
+- reproduction steps;
+- expected impact;
+- logs, proof of concept, or packet captures when relevant;
+- whether the issue is already public.
+
+We aim to acknowledge credible reports within 48 hours and coordinate fixes
+before public disclosure.
+
+## Scope
+
+Security-sensitive areas include:
+
+- authentication, session handling, CSRF, and rate limiting;
+- release and app-catalog signature verification;
+- container manifest validation and runtime compilation;
+- Podman/Quadlet isolation, capabilities, volumes, and secret injection;
+- backup encryption and key derivation;
+- federation, Tor, Nostr, mesh, DID, and credential flows;
+- Android companion pairing and device-token handling.
+
+## Supported versions
+
+Archipelago is currently pre-1.0 alpha software. Security fixes target the
+current `main` branch and the latest published alpha release.