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>

corrosion-host-agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.7"
+version = "2.0.0-alpha.8"
 edition = "2021"
 description = "Corrosion Host Agent — multi-game ops runtime for self-hosted game servers"
 license = "UNLICENSED"