feat(config): env-var loading

Add internal/config package with Config struct and Load() function.
Reads GITEA_BASE_URL, GITEA_API_TOKEN, GITEA_MCP_ALLOWED_OWNERS,
GITEA_MCP_ORIGIN_ALLOWLIST, GITEA_MCP_PORT with sensible defaults.
Wire cfg.Port into main.go. TDD: tests written first, then impl.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -0,0 +1,46 @@
+package config
+
+import (
+	"os"
+	"strings"
+)
+
+type Config struct {
+	Port            string   // GITEA_MCP_PORT, default 8080
+	GiteaBaseURL    string   // GITEA_BASE_URL, e.g. https://gitea.d-ma.be
+	GiteaAPIToken   string   // GITEA_API_TOKEN — bot user token
+	AllowedOwners   []string // GITEA_MCP_ALLOWED_OWNERS, comma-separated, default "mathias"
+	OriginAllowlist []string // GITEA_MCP_ORIGIN_ALLOWLIST, comma-separated
+}
+
+func Load() (Config, error) {
+	cfg := Config{
+		Port:            envOr("GITEA_MCP_PORT", "8080"),
+		GiteaBaseURL:    os.Getenv("GITEA_BASE_URL"),
+		GiteaAPIToken:   os.Getenv("GITEA_API_TOKEN"),
+		AllowedOwners:   splitCSV(envOr("GITEA_MCP_ALLOWED_OWNERS", "mathias")),
+		OriginAllowlist: splitCSV(os.Getenv("GITEA_MCP_ORIGIN_ALLOWLIST")),
+	}
+	return cfg, nil
+}
+
+func envOr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func splitCSV(s string) []string {
+	if s == "" {
+		return nil
+	}
+	parts := strings.Split(s, ",")
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		if p = strings.TrimSpace(p); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}