feat: rework thin wrapper to invoke game-ci/cli as a subprocess

Supersedes the previous approach on this branch, which imported
@game-ci/unity-engine-core as an in-process library. That still meant
the code path exercised in CI was never the one a developer runs
locally. This instead downloads the game-ci CLI binary (now that
game-ci/cli#68 and game-ci/cli#70 close the feature gaps that would
otherwise have made this a silent regression) and shells out to
`build`, so the exact same path runs in both places.

- build-args.ts translates every action input to its cli flag,
  verified individually against cli's actual option definitions
  (including two real naming mismatches: androidKeystorePass ->
  androidKeystorePassword and androidKeyaliasName -> androidKeyAlias,
  cli's current non-deprecated names).
- download-cli.ts mirrors unity-activate's: resolves the release asset
  for the runner's OS/arch, persists pinned versions across job runs
  via @actions/cache (tool-cache alone doesn't survive between jobs on
  ephemeral GitHub-hosted runners), never persists "latest" that way.
- Credentials (UNITY_EMAIL etc.) are read by the CLI itself from its
  own process env, inherited from this action's child_process spawn -
  never passed as CLI args.
- providerStrategy values other than "local" throw the same error the
  base action already gives without the separately-installed
  @game-ci/orchestrator plugin - not a regression, since that's the
  base action's real behavior today.
- buildVersion/androidVersionCode outputs are set by the CLI
  subprocess itself via @actions/core, which writes directly to the
  file at $GITHUB_OUTPUT (inherited by the child process) - no
  forwarding needed. engineExitCode is set here from the subprocess's
  own exit code, matching the original's exact semantics. `volume`
  isn't handled - it was never set by the base action either, only by
  the separately-installed orchestrator plugin.
- action.yml gains a `cliVersion` input (default "latest"). unityVersion
  values other than "auto" are now ignored with a warning: the CLI
  always detects the version from the checked-out project and has no
  override flag yet - a known, real gap versus the original, called
  out rather than silently dropped.
- Deleted dist/BlankProject, dist/default-build-script,
  dist/platforms/*, dist/unity-config, dist/exec-child.js: all dead
  under the new structure. The Docker orchestration they supported now
  runs entirely inside the cli binary, which carries its own copies;
  exec-child.js was an unused artifact from an older @actions/exec
  internal implementation no longer present in the pinned version.

action.yml

@@ -9,7 +9,16 @@ inputs:
   unityVersion:
     required: false
     default: 'auto'
-    description: 'Version of unity to use for building the project. Use "auto" to get from your ProjectSettings/ProjectVersion.txt'
+    description:
+      'Version of unity to use for building the project. Use "auto" to get from your ProjectSettings/ProjectVersion.txt.
+      A value other than "auto" is currently ignored - the underlying game-ci CLI always detects the version from the
+      checked-out project and has no override flag yet.'
+  cliVersion:
+    required: false
+    default: 'latest'
+    description:
+      'Release tag of the game-ci/cli binary to install and run (e.g. "v0.1.0"). Defaults to the latest release; pin
+      this for reproducible builds.'
   customImage:
     required: false
     default: ''
@@ -181,8 +190,7 @@ inputs:
   linux64RemoveExecutableExtension:
     default: 'false'
     required: false
-    description:
-      'When building for StandaloneLinux64, remove the default file extension of `.x86_64`. Set to true to restore the extensionless behavior from v4.'
+    description: 'When building for StandaloneLinux64, remove the default file extension of `.x86_64`. Set to true to restore the extensionless behavior from v4.'
 
 outputs:
   volume: