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>

README.md

@@ -398,6 +398,11 @@ while IFS= read -r dir; do
       ;;
   esac
 done
+# Directories gstack created carry a .gstack-owned marker (the only signal on
+# Windows, where installs are file copies with no symlink to read)
+for marker in ~/.claude/skills/*/.gstack-owned; do
+  [ -f "$marker" ] && rm -rf "$(dirname "$marker")"
+done
 # Alias skills install as copies (no symlink to detect) — remove by name
 rm -rf ~/.claude/skills/_gstack-command ~/.claude/skills/connect-chrome 2>/dev/null
 
@@ -543,6 +548,32 @@ Data is stored in [Supabase](https://supabase.com) (open source Firebase alterna
 
 On Windows without Developer Mode (MSYS2 / Git Bash), `setup` falls back to file copies instead of symlinks because `ln -snf` produces frozen copies that don't refresh on `git pull`. **Re-run `cd ~/.claude/skills/gstack && ./setup` after every `git pull`** so your skill files match the repo. `setup` prints a one-line note reminding you. Unix and WSL keep symlinks and don't need the re-run.
 
+**Chromium install failed or hung during `./setup`?** The bundled browser is
+best-effort: setup records the reason, finishes registering every skill, and
+prints which skills are affected (`/qa`, `/qa-only`, `/design-review`,
+`/browse`, make-pdf, `/diagram`, `/pair-agent`). With Aside open, the browser
+skills keep running in Aside and only the fallback engine is missing;
+`/pair-agent` always needs the bundled browser. Fix the cause and re-run
+`./setup`. Knobs:
+`GSTACK_PLAYWRIGHT_INSTALL_TIMEOUT=<seconds>` raises the download bound
+(default 600) on slow links; `GSTACK_SKIP_PLAYWRIGHT=1` skips the Chromium
+install entirely (CI, no-browser boxes); `GSTACK_CHROMIUM_NO_SANDBOX=1` is the
+fix when Chromium installs but cannot launch because the host blocks
+unprivileged user namespaces (Ubuntu 24.04+ AppArmor default, #2157).
+
+**Setup ended with "Not registered (a skill you own already uses the name; left untouched)"?**
+gstack only deletes or links over a skill entry it can prove it created: a
+symlink into gstack, a directory carrying the `.gstack-owned` marker `./setup`
+writes into every directory it creates, or a SKILL.md that is byte-identical to
+gstack's or carries the generated `<!-- AUTO-GENERATED from ... -->` banner. A
+`qa/` or `ship/` you wrote yourself is left untouched by `./setup`,
+`gstack-relink`, and both prefix-mode flips, and the linker names it in the
+final summary. Rename or move yours, or switch modes (`./setup --prefix` /
+`--no-prefix`) so the names stop colliding. If you started your own skill from
+a generated gstack SKILL.md and then edited it, that file is moved to
+`~/.gstack/backups/skills/<timestamp>/<skill>/SKILL.md` before gstack's is
+linked in, never deleted.
+
 **Claude says it can't see the skills?** Make sure your project's `CLAUDE.md` has a gstack section. Add this:
 
 ```