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>

scripts/test-free-shards.ts

@@ -262,8 +262,8 @@ const KNOWN_WINDOWS_SAFE: Array<{ file: string; reason: string }> = [
   {
     file: 'test/setup-windows-rerun-refresh.test.ts',
     // Trips the "spawns bin/ shebang script" pattern via path.join(..., 'bin',
-    // 'tool.sh') fixture paths, but every spawn goes through spawnSync('bash',
-    // ['-c', ...]) — Git Bash executes it fine on windows-latest. This file IS
+    // 'tool.sh') fixture paths, but every spawn goes through test/helpers/bash-script.ts
+    // (bash <tempfile>) — Git Bash executes it fine on windows-latest, with no argv-length ceiling. This file IS
     // the #2444 Windows regression coverage (IS_WINDOWS=1 copy-refresh path);
     // excluding it here would keep the bug class unexercised on the one
     // platform it bites.