fix(R-1298): held-app reports only for installed apps (a restore hold always)

Red-proved. Commit only.

CHANGELOG.md

@@ -34,6 +34,11 @@ hub answers 400 to both and nothing else changes.
   runs `psql -v ON_ERROR_STOP=1`. Tests `internal/appexport/r1187_abdfg_test.go` (6), red-proved with the old behaviour
   behind the new function shapes (link packing error, a 10-s hang on the FIFO, link not restored, import not refused,
   bad paths accepted, postgis missed).
+- **R-1298:** the hold report skips an install's hold (`unhealthy_stop`, `update_failed`, `putback_pending`) of an app that
+  is not installed (`holdreport.Reporter.SetInstalled`: the app's `app.yaml` exists — an unreadable one still counts,
+  R-1220) and reports it cleared if it was reported; a restore hold is always reported (R-379). 9202's four orphan holds
+  from a hand removal would otherwise have mailed weekly. Test `TestR1298_OnlyHoldsOfInstalledAppsAreReported`,
+  red-proved; the wiring test pins the `SetInstalled` line.
 - `settings.RestoreHold.Why()` / `WireReason()`: the hold sentence moved out of `cmd/controller` (`holdWhy` calls it), so
   `--restore-holds` and the hub's list say the same thing.
 - Tests `internal/holdreport/holdreport_test.go` (8): the report field, the daily repeat, cleared once, cleared while