Refactor Gemini protocol implementation and improve server architecture

- Move gemini URL parsing from common/ to gemini/ package
- Add structured status codes in gemini/status_codes.go
- Improve error handling with proper Gemini status codes
- Update configuration field naming (Listen -> ListenAddr)
- Add UTF-8 validation for URLs
- Enhance security with better path validation
- Add CLAUDE.md for development guidance
- Include example content in srv/ directory
- Update build system to use standard shell

gemini/status_codes.go

@@ -0,0 +1,34 @@
+package gemini
+
+// Gemini status codes as defined in the Gemini spec
+// gemini://geminiprotocol.net/docs/protocol-specification.gmi
+const (
+	// Input group
+	StatusInputExpected          = 10
+	StatusInputExpectedSensitive = 11
+
+	StatusSuccess = 20
+
+	// Redirect group
+	StatusRedirectTemporary = 30
+	StatusRedirectPermanent = 31
+
+	// Temporary failure group
+	StatusTemporaryFailure  = 40
+	StatusServerUnavailable = 41
+	StatusCGIError          = 42
+	StatusProxyError        = 43
+	StatusSlowDown          = 44
+
+	// Permanent failure group
+	StatusPermanentFailure    = 50
+	StatusNotFound            = 51
+	StatusGone                = 52
+	StatusProxyRequestRefused = 53
+	StatusBadRequest          = 59
+
+	// TLS certificate group
+	StatusCertificateRequired      = 60
+	StatusCertificateNotAuthorized = 61
+	StatusCertificateNotValid      = 62
+)