Merge origin/main (v1.80.0.0) into consolidate-browser-skills-into-aside; queue-advance release to v1.81.0.0

Both sides claimed v1.80.0.0, so VERSION, package.json and the agents digest
auto-merged without a conflict; the release is re-versioned to the next free
MINOR slot (bin/gstack-next-version), the size-budget baseline renamed to
match, and the CHANGELOG carries the Aside-first entry above main's.

Two semantic conflicts hidden in the clean setup auto-merge are resolved here:
- _prune_stale_generated deleted a REAL host directory on banner-only proof;
  main's #2119 gate makes that weak proof file-scoped, so real dirs now go
  through _cleanup_weak_dir (SKILL.md, marker and our links only).
- _browser_hint and the Chromium bootstrap summary now consult _PW_FAIL_REASON
  and Aside presence, so they never promise a bundled browser that cannot
  launch and never tell an Aside user their browser skills are gone.
README, TESTING_INTERNALS and TODOS wording reconciled with the merged tree.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.80.0",
+  "version": "1.81.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",