fix: decouple session resume from isFirstVisit guard

When frontmatter validation fails, the step is never written to CAS, so
isFirstVisit remains true on the next run.  Both agent-claude-code and
agent-hermes gated session cache lookup behind !isFirstVisit, which
caused them to start a fresh session (and a new worktree) instead of
resuming the one that already has all the work done.

Changes:
- Remove the isFirstVisit guard from both adapters so they always check
  the session cache.
- When isFirstVisit + cache hit (frontmatter-only failure), send a
  minimal correction prompt via buildFrontmatterRetryPrompt() instead
  of re-sending the full initial prompt — the session already has full
  context, we just need the agent to re-output correctly formatted
  frontmatter.
- Add buildFrontmatterRetryPrompt to util-agent with tests.

Fixes #139

packages/util-agent/src/frontmatter-retry-prompt.ts

@@ -0,0 +1,21 @@
+/**
+ * Build a minimal prompt for retrying frontmatter output on a resumed session.
+ *
+ * Used when a previous run completed successfully but frontmatter validation
+ * failed — the session already has full context, we just need the agent to
+ * re-output correctly formatted frontmatter without redoing any work.
+ */
+export function buildFrontmatterRetryPrompt(outputFormatInstruction: string): string {
+  const parts: string[] = [
+    "Your previous run completed all work successfully, but the output format was incorrect.",
+    "You do NOT need to redo any work — all changes are already in place.",
+    "",
+  ];
+  if (outputFormatInstruction !== "") {
+    parts.push(outputFormatInstruction, "");
+  }
+  parts.push(
+    "Please output ONLY the corrected YAML frontmatter block (--- delimited) followed by a brief summary of the work you completed.",
+  );
+  return parts.join("\n");
+}

packages/util-agent/src/index.ts

@@ -12,6 +12,7 @@ export {
 } from "./extract.js";
 export type { FrontmatterFastPathResult } from "./frontmatter.js";
 export { tryFrontmatterFastPath } from "./frontmatter.js";
+export { buildFrontmatterRetryPrompt } from "./frontmatter-retry-prompt.js";
 export { createAgent, parseArgv } from "./run.js";
 export { getCachedSessionId, getCachePath, setCachedSessionId } from "./session-cache.js";
 export { getConfigPath, getEnvPath, loadWorkflowConfig, resolveStorageRoot } from "./storage.js";