fix(agent): derive @version from BUILD_ID so Burrito re-extracts per build

Burrito keys its on-host install directory on `{release}_erts-{ertsver}_{appver}`
and skips extraction when `_metadata.json` is already present for that version.
With a static `@version "0.1.0"` in mix.exs, every new build landed in the same
cached dir on the target host — silently running stale code.

Now @version resolves to `0.1.0+<BUILD_ID>` where BUILD_ID is the git short SHA
(or `dev-<timestamp>` fallback). scripts/build-linux.sh computes it on the host
and passes it through Dockerfile.build's ARG/ENV, so every commit produces a
distinct Burrito install dir and fresh extraction is guaranteed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent/mix.exs

@@ -1,12 +1,12 @@
 defmodule ProxmoxAgent.MixProject do
   use Mix.Project
 
-  @version "0.1.0"
+  @version_base "0.1.0"
 
   def project do
     [
       app: :agent,
-      version: @version,
+      version: version(),
       elixir: "~> 1.17",
       start_permanent: Mix.env() == :prod,
       deps: deps(),
@@ -22,6 +22,15 @@ defmodule ProxmoxAgent.MixProject do
     ]
   end
 
+  # Version is built per-invocation so every commit produces a distinct Burrito
+  # install directory on the target host (`<release>_erts-<ertsver>_<version>`).
+  # Without this, Burrito sees an existing `_metadata.json` for the same version
+  # and skips extraction, silently running stale cached code.
+  defp version do
+    build_id = System.get_env("BUILD_ID") || "dev"
+    "#{@version_base}+#{build_id}"
+  end
+
   defp deps do
     [
       {:slipstream, "~> 1.1"},