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_test.go

@@ -0,0 +1,38 @@
+package config_test
+
+import (
+	"testing"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/config"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadDefaults(t *testing.T) {
+	t.Setenv("GITEA_BASE_URL", "")
+	t.Setenv("GITEA_API_TOKEN", "")
+	t.Setenv("GITEA_MCP_ALLOWED_OWNERS", "")
+	t.Setenv("GITEA_MCP_ORIGIN_ALLOWLIST", "")
+	t.Setenv("GITEA_MCP_PORT", "")
+
+	cfg, err := config.Load()
+	require.NoError(t, err)
+	assert.Equal(t, "8080", cfg.Port)
+	assert.Equal(t, []string{"mathias"}, cfg.AllowedOwners)
+}
+
+func TestLoadFromEnv(t *testing.T) {
+	t.Setenv("GITEA_BASE_URL", "https://gitea.d-ma.be")
+	t.Setenv("GITEA_API_TOKEN", "test-token")
+	t.Setenv("GITEA_MCP_ALLOWED_OWNERS", "mathias,acme")
+	t.Setenv("GITEA_MCP_ORIGIN_ALLOWLIST", "https://claude.ai,https://api.anthropic.com")
+	t.Setenv("GITEA_MCP_PORT", "9000")
+
+	cfg, err := config.Load()
+	require.NoError(t, err)
+	assert.Equal(t, "https://gitea.d-ma.be", cfg.GiteaBaseURL)
+	assert.Equal(t, "test-token", cfg.GiteaAPIToken)
+	assert.Equal(t, []string{"mathias", "acme"}, cfg.AllowedOwners)
+	assert.Equal(t, []string{"https://claude.ai", "https://api.anthropic.com"}, cfg.OriginAllowlist)
+	assert.Equal(t, "9000", cfg.Port)
+}