fix: stop Doctor exposing hash fragments as unset variables (#1558)

Classify Compose stderr unset-variable warnings into intentional references vs literal-dollar fragments from secret values. Adds env-literal-dollar preflight rule and safe remediation text. Fixes #1550.

backend/src/helpers/envFileResolution.ts

@@ -55,6 +55,8 @@ export interface StackEnvSources {
   inlineEnvKeysByService: Record<string, string[]>;
   /** `${}` references found across the authored compose source. */
   interpolationRefs: InterpolationRef[];
+  /** Concatenated authored compose file text (in-memory classification only). */
+  authoredComposeText: string;
 }
 
 interface EnvFileEntry {
@@ -281,6 +283,7 @@ export async function resolveStackEnvSources(nodeId: number, stackName: string):
     envFiles: [...byPath.values(), ...unresolved],
     inlineEnvKeysByService,
     interpolationRefs: parseInterpolationRefs(authoredText),
+    authoredComposeText: authoredText,
   };
 }