fix(security): close cross-model (Codex) adversarial findings

Codex adversarial review found a HIGH the Claude pass missed plus 3 mediums:
- C1 (HIGH): gstack-decision-search --all returned every decide and IGNORED redact
  events, so a redacted secret still resurfaced via --all until compact ran. --all
  now excludes redacted (redact = expunge from every read path), still showing
  superseded history.
- C-med: semantic (external gbrain) slug/snippet were printed raw — datamark them too
  so a gbrain hit can't spoof role markers / fences into agent context.
- C4: semanticRecall fell back to an UNSCOPED gbrain search when no curated-memory
  source resolved, pulling code/doc corpora mislabeled as 'related decisions'. Now
  returns null (degrade) when there's no worktree-backed memory source.
- C5: validateDecide scanned only decision/rationale/alternatives; branch and issue
  are stored + surfaced (raw via --json), so include them in the injection+secret scan.

C2 (snapshot staleness) / C3 (compact TOCTOU residual): accepted for a single-user
store — atomic appends never lose the event, rebuilds self-heal, and the compact
size-recheck leaves only a sub-ms window; full append-locking would break the
lock-free append design.

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

lib/gstack-decision.ts

@@ -119,7 +119,9 @@ export function validateDecide(input: Partial<DecisionEvent>): ValidateResult {
     }
   }
 
-  const freeText = [input.decision, input.rationale, input.alternatives_considered]
+  // Scan ALL stored free-text — incl. branch/issue, which are surfaced (and emitted raw
+  // via --json), so they must not carry secrets or injection either (Codex finding).
+  const freeText = [input.decision, input.rationale, input.alternatives_considered, input.branch, input.issue]
     .filter((s): s is string => typeof s === "string")
     .join("\n");