merge: incorporate origin/main into community-mode branch

Conflicts resolved:
- README.md: merge skill lists — keep /gstack-submit from our branch,
  add /plan-devex-review, /devex-review, /pair-agent from main. Accept
  main's team mode step 2 text.
- setup: keep both our install ping (step 9) and main's team mode
  hook registration (step 10)
- supabase/functions/telemetry-ingest/index.ts: keep our deletion
  (dead code removed earlier on this branch, main modified it)

Main brought in: team mode (--team flag, auto-update hook, session
tracking), /plan-devex-review + /devex-review skills, /pair-agent
skill, open-gstack-browser, /checkpoint, /health, /humanizer.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-telemetry-sync

@@ -117,6 +117,11 @@ case "$HTTP_CODE" in
     # Advance by SENT count (not inserted count) because we can't map inserted back to
     # source lines. If inserted==0, something is systemically wrong — don't advance.
     INSERTED="$(grep -o '"inserted":[0-9]*' "$RESP_FILE" 2>/dev/null | grep -o '[0-9]*' || echo "0")"
+    # Check for upsert errors (installation tracking failures) — log but don't block cursor advance
+    UPSERT_ERRORS="$(grep -o '"upsertErrors"' "$RESP_FILE" 2>/dev/null || true)"
+    if [ -n "$UPSERT_ERRORS" ]; then
+      echo "[gstack-telemetry-sync] Warning: installation upsert errors in response" >&2
+    fi
     if [ "${INSERTED:-0}" -gt 0 ] 2>/dev/null; then
       NEW_CURSOR=$(( CURSOR + COUNT ))
       echo "$NEW_CURSOR" > "$CURSOR_FILE" 2>/dev/null || true