ci(matrix): 298 skips on aarch64 for the reason the plan documents, and the step said no

`ubuntu-24.04-arm`, with the bootstrap now working end to end:

    295  ok   gcc@16.1.0: naming aarch64-linux-musl changes nothing
    296  ok   the C library the query names is on the link line
    297  ok   ok   ok
    298  SKIP: llvm is not installed here

That skip is correct and is the ecosystem gap itself: upstream LLVM stopped
publishing linux-aarch64 after 19.x, mcpp pins 20.1.7 / 22.1.8, and the index has
no aarch64 llvm. 298 declares llvm, so on that host it has nothing to declare.

The exemption is granted by REASON, mirroring 297's, with linux-x86_64 keeping
the strict check as the denominator. ⭐ Granting it that way makes it retire
itself: the day an aarch64 llvm lands, this stops being a skip and becomes the
assertion, with nothing here to change.

⚠️ Note what 295 measured on that host — the target it resolves is
`aarch64-linux-musl`, not `aarch64-linux-gnu`. mcpp already routes gcc to the
musl payload on non-x86_64 Linux, and the identity holds there.

Author: speak-agent

.github/workflows/ci-target-matrix.yml

@@ -144,8 +144,28 @@ jobs:
               "OK: a capability pin is not a preference" \
               "gcc is not installed here" || fail=1
           fi
-          check 298_overriding_a_convention_requires_replacing_it.sh \
-            "OK: a convention may be overridden, but not merely removed" || fail=1
+          if [ "${{ matrix.host }}" = linux-x86_64 ]; then
+            check 298_overriding_a_convention_requires_replacing_it.sh \
+              "OK: a convention may be overridden, but not merely removed" || fail=1
+          else
+            # ⚠️ THE MIRROR OF 297's EXEMPTION, AND FOR THE OTHER FAMILY.
+            #
+            # 298 declares llvm, and there is no llvm payload for aarch64 Linux —
+            # upstream stopped publishing linux-aarch64 after 19.x and the index
+            # has none. Measured on `ubuntu-24.04-arm`:
+            #
+            #     SKIP: llvm is not installed here, and this test is about
+            #           declaring it
+            #
+            # ⭐ Granting it by REASON rather than by host is what makes it
+            # retire itself: the day
+            # `.agents/docs/2026-08-26-aarch64-linux-ecosystem-closure.md`
+            # lands an aarch64 llvm, this stops being a skip and starts being
+            # the assertion, with nothing here to change.
+            check_or_declared_skip 298_overriding_a_convention_requires_replacing_it.sh \
+              "OK: a convention may be overridden, but not merely removed" \
+              "llvm is not installed here" || fail=1
+          fi
           [ "$fail" = 0 ] || exit 1
 
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