feat(worker): add Claude CLI preflight on startup

Worker now runs `claude --version` before listening; on non-zero exit
it logs critical and exits with code 1. Skippable via env var
CLAUDEDO_SKIP_CLI_PREFLIGHT=1 for environments without the CLI (tests,
dev). Closes verification step 2 / open.md item 3.1.

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

src/ClaudeDo.Worker/Lifecycle/ClaudeCliPreflight.cs

@@ -0,0 +1,38 @@
+using System.Diagnostics;
+
+namespace ClaudeDo.Worker.Lifecycle;
+
+public static class ClaudeCliPreflight
+{
+    public sealed record Result(bool Ok, string Version, string Error, int ExitCode);
+
+    public static async Task<Result> CheckAsync(string claudeBin, CancellationToken ct = default)
+    {
+        try
+        {
+            var psi = new ProcessStartInfo
+            {
+                FileName = claudeBin,
+                Arguments = "--version",
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true,
+            };
+            using var proc = Process.Start(psi);
+            if (proc is null) return new Result(false, "", "Process.Start returned null", -1);
+
+            var stdoutTask = proc.StandardOutput.ReadToEndAsync(ct);
+            var stderrTask = proc.StandardError.ReadToEndAsync(ct);
+            await proc.WaitForExitAsync(ct);
+
+            var stdout = (await stdoutTask).Trim();
+            var stderr = (await stderrTask).Trim();
+            return new Result(proc.ExitCode == 0, stdout, stderr, proc.ExitCode);
+        }
+        catch (Exception ex)
+        {
+            return new Result(false, "", ex.Message, -1);
+        }
+    }
+}