chore: bump version and changelog (v1.13.1.0)

Slim preamble work + real-PTY plan-mode E2E harness on top of v1.13.0.0.
SKILL.md corpus -25.5% (3.08 MB → 2.30 MB, ~196K tokens). 5 plan-mode
tests go from 0/5 to 5/5 (790s sequential), the first time those tests
have ever passed. Side-fixes for the 27MB security fixture warning and
the sidecar-symlink double-count.

Reverts the Fan-Out directive accidentally restored to opus-4-7.md —
v1.10.1.0's overlay-efficacy harness measured -60pp fanout vs baseline
when the nudge was active. The intentional removal stays.

TODOS:
- Pre-existing test failures from v1.12.0.0 ship: RESOLVED on main + this branch
- security-bench-haiku-responses.json size gate: RESOLVED via warn-only + exemption

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.13.0.0",
+  "version": "1.13.1.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",