feat: add cross-platform release automation

Add a GitHub Actions workflow that validates the app on pushes and pull requests, and publishes Windows, macOS, and Linux release assets when a tag is pushed.

Also replace the Windows-only packaging flow with cross-platform Bun scripts for sidecar publishing and Electron packaging so the release job can package each runner natively.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

scripts/publish-sidecar.ts

@@ -0,0 +1,65 @@
+import { spawn } from 'node:child_process';
+import { rm } from 'node:fs/promises';
+import { dirname, join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { resolveReleaseTarget } from './releaseTarget';
+
+function runCommand(command: string, args: string[], cwd: string): Promise<void> {
+  return new Promise((resolvePromise, rejectPromise) => {
+    const child = spawn(command, args, {
+      cwd,
+      stdio: 'inherit',
+    });
+
+    child.on('error', rejectPromise);
+    child.on('exit', (code, signal) => {
+      if (code === 0) {
+        resolvePromise();
+        return;
+      }
+
+      if (signal) {
+        rejectPromise(new Error(`${command} exited because of signal ${signal}.`));
+        return;
+      }
+
+      rejectPromise(new Error(`${command} exited with code ${code ?? 'unknown'}.`));
+    });
+  });
+}
+
+const scriptDirectory = dirname(fileURLToPath(import.meta.url));
+const repositoryRoot = resolve(scriptDirectory, '..');
+const releaseTarget = resolveReleaseTarget(process.platform, process.arch);
+const sidecarProjectPath = join(
+  repositoryRoot,
+  'sidecar',
+  'src',
+  'Eryx.AgentHost',
+  'Eryx.AgentHost.csproj',
+);
+const outputDirectory = join(repositoryRoot, 'dist-sidecar', releaseTarget.dotnetRuntime);
+
+await rm(outputDirectory, { recursive: true, force: true });
+
+await runCommand(
+  'dotnet',
+  [
+    'publish',
+    sidecarProjectPath,
+    '-c',
+    'Release',
+    '-r',
+    releaseTarget.dotnetRuntime,
+    '--self-contained',
+    'true',
+    '-p:DebugType=None',
+    '-p:DebugSymbols=false',
+    '-o',
+    outputDirectory,
+  ],
+  repositoryRoot,
+);
+
+console.log(`Published sidecar for ${releaseTarget.platformLabel} (${releaseTarget.dotnetRuntime}) to ${outputDirectory}`);