docs: add SECURITY.md with private vulnerability reporting policy

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SECURITY.md

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+# Security Policy
+
+Amethyst is a Nostr client that handles user private keys, signed events, and
+end-to-end encrypted direct messages. We take security reports seriously and
+appreciate responsible disclosure.
+
+## Supported Versions
+
+Only the latest release receives security fixes. We do not backport patches to
+older versions. Fixes also land on the `main` branch ahead of the next release.
+
+| Version        | Supported |
+| -------------- | --------- |
+| Latest release | ✅        |
+| `main`         | ✅        |
+| Older          | ❌        |
+
+This covers all artifacts built from this repository: the Android app
+(`amethyst/`), the desktop app (`desktopApp/`), the `amy` CLI (`cli/`), and
+the `quartz` / `commons` libraries.
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Use GitHub's private vulnerability reporting instead:
+
+👉 [Report a vulnerability](https://github.com/vitorpamplona/amethyst/security/advisories/new)
+
+This keeps the details private until a fix is ready and coordinates disclosure
+between you and the maintainers.
+
+### What to include
+
+To help us triage quickly, please provide:
+
+- A clear description of the vulnerability and its impact (what an attacker
+  could achieve — e.g. key material exposure, DM confidentiality, integrity,
+  availability).
+- Affected module(s): `quartz`, `commons`, `amethyst` (Android), `desktopApp`,
+  or `cli` / `amy`.
+- Affected version(s), commit SHA, platform, and OS.
+- Steps to reproduce, a proof of concept, or a failing test.
+- Any suggested remediation.
+
+### What to expect
+
+- **Acknowledgement within 48 hours** of your report.
+- We will investigate and keep you informed of progress.
+- We will coordinate a release and disclosure timeline with you. We aim to
+  ship a fix within 90 days for high and critical issues, faster when key
+  material or DM confidentiality is at risk.
+- Credit will be given to reporters in the security advisory and release
+  notes (unless you prefer to remain anonymous).
+
+## Scope
+
+In scope:
+
+- Source code in this repository across all modules.
+- Released binaries (APK, DMG, MSI, DEB, RPM, AppImage, tarball) built from
+  this repository.
+- Cryptographic handling: signing, NIP-04 / NIP-17 / NIP-44 encryption, key
+  storage (Android Keystore, desktop keychain), NIP-46 bunker flows, NIP-55
+  external signer integration.
+- Relay client behavior that could leak private data or bypass authorization.
+
+Out of scope:
+
+- Vulnerabilities in third-party relays, bridges, media servers, or Nostr
+  clients not built from this repository.
+- Issues that require a rooted / jailbroken device, a compromised host, or
+  physical access with the device unlocked.
+- Weaknesses inherent to the Nostr protocol itself — please report these
+  upstream at <https://github.com/nostr-protocol/nips>.
+- Denial-of-service from a malicious relay the user has explicitly connected
+  to.
+- Social-engineering and phishing that does not exploit an app-level flaw.
+
+## Disclosure Policy
+
+We follow a coordinated disclosure model. We ask that you:
+
+- Give us reasonable time to investigate and release a fix before any public
+  disclosure.
+- Avoid accessing or modifying other users' data during research.
+- Only interact with accounts and data you own or have explicit permission to
+  test.
+- Act in good faith.
+
+We will not pursue or support legal action against researchers who follow
+this policy. We commit to responding promptly and treating all reports
+seriously.
+
+Thank you for helping keep Amethyst and its users safe.