Add nurl and vainstr CLI tools (v0.39.0)

- Add nurl: NIP-98 authenticated HTTP client for testing owner APIs
- Add vainstr: vanity npub generator using fast secp256k1 library
- Update CLAUDE.md with documentation for both tools
- Properly handle secp256k1 library loading via p8k.New()

Files modified:
- cmd/nurl/main.go: New NIP-98 HTTP client tool
- cmd/vainstr/main.go: New vanity npub generator
- CLAUDE.md: Added usage documentation for nurl and vainstr
- go.mod/go.sum: Added go-arg dependency for vainstr
- pkg/version/version: Bump to v0.39.0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

go.mod

@@ -31,6 +31,8 @@ require (
 require (
 	github.com/BurntSushi/toml v1.5.0 // indirect
 	github.com/ImVexed/fasturl v0.0.0-20230304231329-4e41488060f3 // indirect
+	github.com/alexflint/go-arg v1.6.1 // indirect
+	github.com/alexflint/go-scalar v1.2.0 // indirect
 	github.com/btcsuite/btcd/btcec/v2 v2.3.4 // indirect
 	github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0 // indirect
 	github.com/bytedance/sonic v1.13.1 // indirect