feat: add Nostr channel plugin and onboarding install defaults

Co-authored-by: joelklabo <joelklabo@users.noreply.github.com>

extensions/nostr/CHANGELOG.md

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+# Changelog
+
+## 2026.1.19-1
+
+Initial release.
+
+### Features
+
+- NIP-04 encrypted DM support (kind:4 events)
+- Key validation (hex and nsec formats)
+- Multi-relay support with sequential fallback
+- Event signature verification
+- TTL-based deduplication (24h)
+- Access control via dmPolicy (pairing, allowlist, open, disabled)
+- Pubkey normalization (hex/npub)
+
+### Protocol Support
+
+- NIP-01: Basic event structure
+- NIP-04: Encrypted direct messages
+
+### Planned for v2
+
+- NIP-17: Gift-wrapped DMs
+- NIP-44: Versioned encryption
+- Media attachments