ci(host-agent): bootstrap Rust in the runner container; roll to alpha.2

Asgard runner executes jobs in bare node:20-bullseye (no Rust, no sudo)
- install rustup + musl/mingw cross toolchains per-run, same pattern as
setup-go in the Go pipeline. agent-v2.0.0-alpha.1 predates this fix;
forward-only doctrine: version rolls to alpha.2 rather than re-pushing
the tag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.1"
+version = "2.0.0-alpha.2"
 edition = "2021"
 description = "Corrosion Host Agent — multi-game ops runtime for self-hosted game servers"
 license = "UNLICENSED"