PASS 3: add future-chapter contamination guard to SECRET CARRY-FORWARD RULE

When writing ch-07, PASS 3 was reading character-state-ch-08 deliverables and
carrying 'Known secrets' from future chapters into the current state. Added
explicit FUTURE-CHAPTER CONTAMINATION GUARD: only use secrets from state files
whose chapter number is EARLIER than the current chapter being written.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

templates/book_chapter.yml

@@ -363,6 +363,12 @@ steps:
       ══ SECRET CARRY-FORWARD RULE ══
       Before submitting, check the PASS 0 context for any "Known secrets:" fields or
       "CARRIED SECRET" markers from the PREVIOUS character-state file.
+      ⚠ FUTURE-CHAPTER CONTAMINATION GUARD: ONLY carry secrets from character-state files
+        whose filename indicates a chapter EARLIER than {chapter_ref}.
+        Example: if writing ch-07, only use secrets from ch-06, ch-05, ch-04, etc.
+        NEVER copy a secret from character-state-ch-08 or later — those are FUTURE events.
+        If a secret in PASS 0 context comes from a deliverable filename like
+        "character-state-ch-08" or "character-state-ch-09", IGNORE it entirely.
       For each secret that was NOT resolved or revealed in THIS chapter's PASS 1 prose:
         → Copy it forward unchanged into the character's "Known secrets:" field.
         → Prefix it: "CARRIED (Ch[N]—unresolved): [secret]"