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>

tests/ClaudeDo.Worker.Tests/Lifecycle/ClaudeCliPreflightTests.cs

@@ -0,0 +1,29 @@
+using ClaudeDo.Worker.Lifecycle;
+using Xunit;
+
+namespace ClaudeDo.Worker.Tests.Lifecycle;
+
+public sealed class ClaudeCliPreflightTests
+{
+    [Fact]
+    public async Task NonExistentBinary_ReturnsNotOk_WithError()
+    {
+        var result = await ClaudeCliPreflight.CheckAsync("definitely-not-a-real-bin-xyz123");
+
+        Assert.False(result.Ok);
+        Assert.NotEqual(0, result.ExitCode);
+        Assert.NotEmpty(result.Error);
+    }
+
+    [Fact]
+    public async Task BinaryAcceptingVersionFlag_ReturnsOk_WithVersionOutput()
+    {
+        // `dotnet --version` is always available in this build environment, exits 0,
+        // and prints the SDK version on stdout — same shape as `claude --version`.
+        var result = await ClaudeCliPreflight.CheckAsync("dotnet");
+
+        Assert.True(result.Ok, $"expected ok, got exit={result.ExitCode} err='{result.Error}'");
+        Assert.Equal(0, result.ExitCode);
+        Assert.NotEmpty(result.Version);
+    }
+}