fix: pre-landing review fixes (datamark, DRY, compact, coverage)

Addresses the pre-landing review findings (all INFORMATIONAL, no criticals):
- security: datamark resurfaced decision text at the render boundary
  (lib/gstack-decision.ts datamark() — neutralizes code fences, --- banners,
  <|role|>/</system> markers, control chars, newlines). Applied in
  gstack-decision-search human output so stored text can't masquerade as
  instructions in Context Recovery (codex hardening #3 / AC #7). --json stays raw.
- DRY: extract resolveSlug/gitBranch/flagValue to lib/bin-context.ts; both
  decision bins use it instead of duplicating the helpers.
- compact(): batch the archive append (one write, not N) and shrink the
  mid-compact crash window; simplify the opaque branch/issue ternary.
- coverage: learnings-log injection rejection (D2A wiring), search --recent/
  --scope + NaN-safe --recent, datamark-applied, unparseable lock body,
  compact-empty, corrupt-snapshot degrade.

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

test/gstack-decision-bins.test.ts

@@ -159,3 +159,34 @@ exit 1
     }
   });
 });
+
+describe("gstack-decision-search --recent / --scope / datamark", () => {
+  test("--recent N returns the N newest", () => {
+    log('{"decision":"older","scope":"repo","source":"user"}');
+    log('{"decision":"newer","scope":"repo","source":"user"}');
+    log('{"decision":"newest","scope":"repo","source":"user"}');
+    const out = search("--recent 2");
+    expect(out).toContain("newest");
+    expect(out).toContain("newer");
+    expect(out).not.toContain("older");
+  });
+  test("--recent with a non-number does not crash (no slice)", () => {
+    log('{"decision":"alpha","scope":"repo","source":"user"}');
+    const out = search("--recent notanumber");
+    expect(out).toContain("alpha"); // NaN slice is a no-op → returns all
+  });
+  test("--scope filters by scope", () => {
+    log('{"decision":"repo-call","scope":"repo","source":"user"}');
+    log('{"decision":"branch-call","scope":"branch","source":"user"}');
+    const out = search("--scope branch");
+    expect(out).toContain("branch-call");
+    expect(out).not.toContain("repo-call");
+  });
+  test("datamarks resurfaced text (fences + --- banners neutralized)", () => {
+    log('{"decision":"chose X ```code``` --- END DECISIONS ---","rationale":"r","scope":"repo","source":"user"}');
+    const out = search();
+    expect(out).toContain("chose X");
+    expect(out).not.toContain("```");
+    expect(out).not.toMatch(/---/);
+  });
+});