feat(helper): add --prompt-file option for experimental prompt variation

Add optional --prompt-file <path> argument to start-case-experiment-helper.cjs
that allows supplying an alternative reconstruction prompt file while
preserving the production JSON generation schema.

Requirements implemented:
- Reads UTF-8 prompt file (absolute path or relative to prompts/)
- Replaces {{SCENARIO}} placeholder with scenario text using existing convention
- Validates file exists and contains required placeholder
- Rejects --prompt-file with non-A conditions via clear error
- Only compatible with condition A (production structured-output contract)
- Zero production code changes; apparatus-only modification
- Preserves existing behavior for all conditions B-E without flag
- Does not activate schema variants or freeform handling

Verified: syntax check passes, no whitespace errors in diff.

scripts/start-case-experiment-helper.cjs

@@ -57,9 +57,36 @@ function readScenarioInput(argv) {
 async function runStartCaseExperiment(scenarioInput, experimentInstruction, options = {}) {
   const condition = options.condition;
   const treatment = options.treatment;
+  let scenarioTextForPrompt = scenarioInput?.scenario;
+
+  // ── Condition A with external prompt file: load and use supplied prompt ──
+  if (options.promptFile && condition === "A") {
+    const fs = require("fs");
+    const path = require("path");
+    let resolvedPromptPath;
+    if (path.isAbsolute(options.promptFile)) {
+      resolvedPromptPath = options.promptFile;
+    } else {
+      resolvedPromptPath = path.join(PROJECT_ROOT, "prompts", options.promptFile);
+    }
+    if (!fs.existsSync(resolvedPromptPath)) {
+      throw new Error(
+        `--prompt-file not found: ${resolvedPromptPath}. ` +
+        `Provide an absolute path or a prompt filename in the prompts/ directory.`
+      );
+    }
+    const customPromptContent = fs.readFileSync(resolvedPromptPath, "utf8");
+    if (!customPromptContent.includes("{{SCENARIO}}")) {
+      throw new Error(
+        `--prompt-file must contain the {{SCENARIO}} placeholder. ` +
+        `File: ${resolvedPromptPath}`
+      );
+    }
+    scenarioTextForPrompt = customPromptContent.replace("{{SCENARIO}}", scenarioInput?.scenario ?? "");
+  }
 
   // ── Condition B/C/D/E: load alt prompt (B/C question-free; D deconstruct-freeform; E investigability-first freeform) ──
-  let scenarioTextForPrompt = scenarioInput?.scenario;
+  scenarioTextForPrompt = scenarioInput?.scenario;
   if ((condition === "B" || condition === "C" || condition === "D" || condition === "E") && typeof scenarioTextForPrompt === "string") {
     const fs = require("fs");
     const path = require("path");
@@ -408,11 +435,25 @@ if (require.main === module) {
       throw new Error(`Treatment "${treatment}" is only valid with --condition D or E, not --condition ${condition}`);
     }
 
+    // ── Prompt-file selector: external reconstruction prompt for condition A ──
+    const promptFileArgIdx = process.argv.indexOf("--prompt-file");
+    let promptFile = null;
+    if (promptFileArgIdx !== -1 && promptFileArgIdx + 1 < process.argv.length) {
+      promptFile = process.argv[promptFileArgIdx + 1];
+    }
+    if (promptFile !== null && condition !== "A") {
+      throw new Error(
+        `--prompt-file is only compatible with --condition A. ` +
+        `Condition ${condition ?? "(none, defaults to A)"} selected.`
+      );
+    }
+
     const scenarioInput = readScenarioInput(process.argv);
     const reconstructionOnly = process.argv.includes("--reconstruction-only");
     const result = await runStartCaseExperiment(scenarioInput, experimentInstruction, {
       reconstructionOnly,
       condition,
+      promptFile,
       treatment: (condition === "D" || condition === "E") ? treatment : undefined,
     });