docs(upstream-sync): the inventory names docs-sync, which #201 makes forge-deciding (#200)

Found by combining all five open PRs and running the suite on the result —
which is the check this PR's own runbook adds, catching a real break the first
time it was applied at scale.

!203 (#201) makes actions/docs-sync/docs-sync.sh decide the forge from
GITHUB_SERVER_URL, because it was fetching the doctrine mirror from a
hard-coded github.com. This PR's guard requires every forge-deciding file to be
named in the inventory. Both are individually green; together the tree is red:

  forge-specific but not in docs/UPSTREAM-SYNC.md:
    actions/docs-sync/docs-sync.sh

The entry belongs here rather than in !203: the inventory is this PR's artifact,
and !203 is a bug fix that should not have to know about a guard absent from
its base. Adding it early is harmless — the guard checks that deciding files
ARE listed, not that listed files decide — and correct the moment both land.

Five-way combined tree after this: 28 test files 0 failed under the runner's
jq 1.6, shellcheck 0.10.0, actionlint, self-ref, marker, vendored and
changelog-armed all clean.

Refs #200

docs/UPSTREAM-SYNC.md

@@ -253,6 +253,7 @@ by judgement, not by scan.
 | `.github/workflows/labels.yml` | the sweep dispatch decides the forge inline and declares a client; a workflow has no shell to call `forge_preflight` from. #205 ports it |
 | `.github/workflows/refs-guard.yml` | schedules its job on GitHub only, so an action that can only refuse here does not stand red. #199 removes the gate |
 | `.github/workflows/release-exercise.yml` | pins `CEREMONY_FORGE: github` deliberately: the exercise drives the GitHub path |
+| `actions/docs-sync/docs-sync.sh` | fetches the doctrine mirror from the forge in `GITHUB_SERVER_URL`, and refuses rather than guessing one (#201) |
 
 Four of those are **temporary** and say which issue removes them. That is the
 point of listing them rather than exempting them: a forge-delta location with