fix(ci): base64-decode minisign secret key (CI mangles multi-line); bump alpha.8

The 'Sign artifacts' step failed on alpha.7 with 'Error while loading the
secret key file' (exit 2): minisign downloaded and ran, but the reconstructed
key file was unparseable. A minisign secret key is two lines (comment + base64
blob); Gitea/act_runner secret storage mangles the embedded newline, collapsing
it to one line. Decode the secret as base64 (single-line, mangling-proof) with
auto-detect fallback to a raw two-line key. Fails loudly with the fix command
if the secret is neither form.

Requires re-storing MINISIGN_SECRET_KEY as: base64 < secret.key | tr -d '\n'

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-host-agent.yml

@@ -81,7 +81,21 @@ jobs:
           MINISIGN="$(find /tmp -type f -name minisign -path '*linux*' | head -1)"
           chmod +x "$MINISIGN"
           "$MINISIGN" -v
-          printf '%s\n' "$MINISIGN_SECRET_KEY" > /tmp/sign.key
+          # A minisign secret key file is TWO lines (comment + base64 blob). CI
+          # secret storage mangles embedded newlines, collapsing it to one line
+          # so minisign can't load it. Preferred form: store the secret
+          # base64-encoded (single line) — we decode it here. Auto-detect so a
+          # correctly-stored raw two-line key still works.
+          if printf '%s' "$MINISIGN_SECRET_KEY" | base64 -d 2>/dev/null | head -1 | grep -q "untrusted comment:"; then
+            printf '%s' "$MINISIGN_SECRET_KEY" | base64 -d > /tmp/sign.key
+          else
+            printf '%s\n' "$MINISIGN_SECRET_KEY" > /tmp/sign.key
+          fi
+          if ! head -1 /tmp/sign.key | grep -q "untrusted comment:"; then
+            echo "::error::MINISIGN_SECRET_KEY is neither base64 of a minisign key nor a raw two-line key file. Store it as:  base64 < your-secret.key | tr -d '\n'"
+            rm -f /tmp/sign.key
+            exit 1
+          fi
           cd corrosion-host-agent/bin
           # Passwordless key (-W generated); feed empty stdin so it never blocks.
           for f in corrosion-host-agent-linux-amd64 corrosion-host-agent-windows-amd64.exe checksums.txt; do