feat(gitea): support extra outbound HTTP headers via GITEA_EXTRA_HEADERS

Add a GITEA_EXTRA_HEADERS environment variable that accepts a JSON
object of header name/value pairs (e.g. Cloudflare Access
credentials) and applies them to every outbound request to Gitea,
both the raw pkg/gitea.DoJSON/DoBytes path and the SDK-backed
pkg/gitea.NewClient path, without overriding Authorization,
Content-Type, or Accept.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

pkg/gitea/gitea.go

@@ -30,6 +30,32 @@ func sharedTransport() *http.Transport {
 	return sharedTrans
 }
 
+// extraHeaderTransport injects flag.ExtraHeaders into every request, without
+// overriding headers the caller already set (e.g. Authorization, Content-Type,
+// Accept). It reads flag.ExtraHeaders on each round trip rather than caching
+// it, so tests can change it between requests.
+type extraHeaderTransport struct {
+	base http.RoundTripper
+}
+
+func (t *extraHeaderTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	headers := flag.ExtraHeaders
+	if len(headers) == 0 {
+		return t.base.RoundTrip(req)
+	}
+	cloned := req.Clone(req.Context())
+	for name, values := range headers {
+		if cloned.Header.Get(name) == "" {
+			cloned.Header[name] = values
+		}
+	}
+	return t.base.RoundTrip(cloned)
+}
+
+func giteaTransport() http.RoundTripper {
+	return &extraHeaderTransport{base: sharedTransport()}
+}
+
 // NewClient returns a cached *gitea.Client keyed by host+token. The SDK's per-client
 // version cache and the shared transport let us reuse keep-alive connections
 // and avoid the SDK's /api/v1/version preflight on every tool call.
@@ -40,7 +66,7 @@ func NewClient(token string) (*gitea.Client, error) {
 	}
 
 	httpClient := &http.Client{
-		Transport:     sharedTransport(),
+		Transport:     giteaTransport(),
 		CheckRedirect: checkRedirect,
 	}
 	opts := []gitea.ClientOption{