fix(ci): pin the cluster CA in the repo rather than in a secret

The CA fingerprint is not a secret.  It is a public value that anyone can
read straight off the certificate, so pinning it in a Gitea secret was the
wrong shape:

- A change to it would be invisible - no commit, no review.
- Nobody with secret-write access could silently repoint the CI runner at a
  different CA.

It now lives in `scripts/cluster-ca.sha256`, so changing it is a reviewable
commit.  The script reads it from there, and still accepts an override as
its first argument.

This also removes the need for a `KUBE_SERVER_CA_FINGERPRINT` secret, which
the bot token cannot create anyway - the Actions secrets API is owner-only.

The behaviour is unchanged: the CA is fetched over an unverified connection
but only USED if it matches the pinned fingerprint, so an intercepted CA is
refused.  Re-tested against the live API server - the matching fingerprint
refreshes the CA and kubectl then works; a wrong one is refused and the
stale CA is left in place.

scripts/kube-ca-refresh.sh

@@ -12,13 +12,21 @@
 #
 # The refresh is PINNED: the server's CA is fetched over an unverified
 # connection, but it is only used if its SHA-256 fingerprint matches the value
-# in the KUBE_SERVER_CA_FINGERPRINT secret.  An attacker able to intercept
-# the fetch cannot use it - their CA will not hash to the pinned fingerprint.
+# pinned in scripts/cluster-ca.sha256.  An attacker able to intercept the
+# fetch cannot use it - their CA will not hash to the pinned fingerprint.
 # The runner has cluster-admin credentials, so trusting an unverified CA here
 # would be a privilege-escalation hole.
 #
+# The fingerprint is a PUBLIC value - it can be read straight off the
+# certificate - so it lives in the repo rather than in a secret.  That way a
+# change to it is a reviewable commit, and nobody with secret-write access
+# can silently repoint the CI runner at a different CA.
+#
+# The expected fingerprint is read from scripts/cluster-ca.sha256 next to
+# this script.  It can be overridden with the first argument.
+#
 # Usage:
-#   kube-ca-refresh.sh <expected-fingerprint>
+#   kube-ca-refresh.sh [expected-fingerprint]
 #
 # Exit codes:
 #   0  the cluster is reachable
@@ -26,7 +34,12 @@
 
 set -euo pipefail
 
+SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 EXPECTED="${1:-}"
+if [ -z "$EXPECTED" ] && [ -f "$SCRIPT_DIR/cluster-ca.sha256" ]; then
+    # The file may carry comments; take the last non-comment line.
+    EXPECTED=$(grep -vE '^[[:space:]]*(#|$)' "$SCRIPT_DIR/cluster-ca.sha256" | tail -1 | tr -d '[:space:]')
+fi
 
 if kubectl version --request-timeout=15s >/dev/null 2>&1; then
     echo "cluster reachable"
@@ -36,8 +49,8 @@ fi
 if [ -z "$EXPECTED" ]; then
     echo "ERROR: the kubeconfig cannot verify the API server, and no expected CA" >&2
     echo "  fingerprint was supplied, so the CA cannot be refreshed safely." >&2
-    echo "  Set KUBE_SERVER_CA_FINGERPRINT to the SHA-256 fingerprint of the" >&2
-    echo "  cluster's CA:" >&2
+    echo "  and no pinned CA fingerprint could be found." >&2
+    echo "  Set one in scripts/cluster-ca.sha256:" >&2
     echo "    openssl x509 -in server-ca.crt -noout -fingerprint -sha256" >&2
     exit 1
 fi
@@ -72,7 +85,7 @@ echo "actual   CA fingerprint: $ACTUAL"
 if [ "$ACTUAL" != "$WANTED" ]; then
     echo "ERROR: the cluster CA does not match the pinned fingerprint." >&2
     echo "  Refusing to use it.  Either the CA has been rotated and the" >&2
-    echo "  fingerprint secret needs updating, or the connection was" >&2
+    echo "  pinned fingerprint needs updating, or the connection was" >&2
     echo "  intercepted." >&2
     exit 1
 fi