docs: name the update check's conditional compare-API request

bin/gstack-update-check also GETs api.github.com's compare endpoint when the
remote VERSION equals the local one but main has moved; the README privacy
summary and docs/privacy.md now list it beside the ls-remote and VERSION
fetch (documentation audit).

README.md

@@ -256,7 +256,7 @@ On Windows without Developer Mode (MSYS2 / Git Bash), `setup` falls back to file
 
 ## Privacy & Telemetry
 
-gstack sends nothing about how you use it unless you opt in. Telemetry is off by default and asked about once; the update check (two GitHub requests, a `git ls-remote` and a fetch of `VERSION`; with telemetry on it also reports your gstack version and OS; on by default) turns off with `gstack-config set update_check false`; outside reviews send code to Codex or Claude Code under your own login and turn off with `gstack-config set codex_reviews disabled`. Every off-machine send writes a receipt you can audit with `gstack-egress list`. The full account, including the exact fields telemetry carries and the local state that is not telemetry: [docs/privacy.md](docs/privacy.md).
+gstack sends nothing about how you use it unless you opt in. Telemetry is off by default and asked about once; the update check (two or three GitHub requests: a `git ls-remote`, a fetch of `VERSION` and, when that version equals yours but `main` has moved, a compare-API call; with telemetry on it also reports your gstack version and OS; on by default) turns off with `gstack-config set update_check false`; outside reviews send code to Codex or Claude Code under your own login and turn off with `gstack-config set codex_reviews disabled`. Every off-machine send writes a receipt you can audit with `gstack-egress list`. The full account, including the exact fields telemetry carries and the local state that is not telemetry: [docs/privacy.md](docs/privacy.md).
 
 ## Troubleshooting
 

docs/privacy.md

@@ -33,9 +33,11 @@ no, and `gstack-config set telemetry off` turns it off at any time.
 skill start runs `gstack-update-check`. When its cache is stale (60 minutes after an
 up-to-date result, 12 hours while an upgrade is pending, 10 minutes after a
 failure) it asks GitHub for the latest version: a `git ls-remote` of
-`github.com/garrytan/gstack` and a GET of the `VERSION` file from
-`raw.githubusercontent.com`. GitHub sees your IP address; nothing about you or
-your code is sent. With telemetry on, the same check also posts your gstack
+`github.com/garrytan/gstack`, a GET of the `VERSION` file from
+`raw.githubusercontent.com` and, when that version equals yours but `main` has
+moved past your checkout, a GET of `api.github.com`'s compare endpoint for the
+two commit SHAs. GitHub sees your IP address; nothing about you or your code is
+sent. With telemetry on, the same check also posts your gstack
 version and OS to gstack's Supabase. Turn the check off with
 `gstack-config set update_check false`.