feat(auth): document caller header precedence + warn on conflict (#10)

CallerMiddleware silently preferred X-Auth-Request-User over X-Forwarded-User
with no explanation and no signal when both were set. Documented the precedence
(X-Auth-Request-User is the verified OIDC identity oauth2-proxy sets, so it is
authoritative; X-Forwarded-User is a fallback), and it now takes a *slog.Logger
and warns when both headers are present and disagree, so a proxy
misconfiguration is visible instead of silently resolved. Table-driven tests
cover precedence (both/single/none) and the conflict-warning path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/pr_create_test.go

@@ -30,7 +30,7 @@ const prFixture = `{
 
 func callerContext(user string) context.Context {
 	var capturedCtx context.Context
-	h := auth.CallerMiddleware(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+	h := auth.CallerMiddleware(nil, http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
 		capturedCtx = r.Context()
 	}))
 	req := httptest.NewRequest("POST", "/", nil)