feat(mcp): add save_finding to the external server

src/ClaudeDo.Worker/Git/GitExcludeWriter.cs

@@ -13,7 +13,7 @@ public static class GitExcludeWriter
     /// <summary>Idempotent — an already-present line is left alone.</summary>
     public static async Task AppendLineAsync(string workingDir, string excludeLine, CancellationToken ct)
     {
-        var relativeExcludePath = await RunGitCaptureAsync(workingDir, ["rev-parse", "--git-path", "info/exclude"], ct);
+        var relativeExcludePath = await RevParseAsync(workingDir, ["rev-parse", "--git-path", "info/exclude"], ct);
         var excludeFile = Path.IsPathRooted(relativeExcludePath)
             ? relativeExcludePath
             : Path.GetFullPath(Path.Combine(workingDir, relativeExcludePath));
@@ -32,7 +32,13 @@ public static class GitExcludeWriter
         await File.AppendAllTextAsync(excludeFile, excludeLine + Environment.NewLine, ct);
     }
 
-    private static async Task<string> RunGitCaptureAsync(string workingDir, IEnumerable<string> args, CancellationToken ct)
+    /// <summary>
+    /// Runs a `git` invocation and captures stdout. Named for its most common caller
+    /// (rev-parse), but shared plumbing for any short git query that needs one line of output —
+    /// non-zero exit throws <see cref="InvalidOperationException"/>. Shared by AppendLineAsync
+    /// above and by GitHead.ShortAsync.
+    /// </summary>
+    internal static async Task<string> RevParseAsync(string workingDir, IEnumerable<string> args, CancellationToken ct)
     {
         var psi = new ProcessStartInfo
         {
@@ -67,7 +73,8 @@ public static class GitExcludeWriter
         ct.ThrowIfCancellationRequested();
 
         if (proc.ExitCode != 0)
-            throw new InvalidOperationException($"git rev-parse --git-path failed (exit {proc.ExitCode}): {stderr.TrimEnd()}");
+            throw new InvalidOperationException(
+                $"git {string.Join(' ', args)} failed (exit {proc.ExitCode}): {stderr.TrimEnd()}");
 
         return stdout.TrimEnd('\r', '\n');
     }

src/ClaudeDo.Worker/Git/GitHead.cs

@@ -0,0 +1,21 @@
+namespace ClaudeDo.Worker.Git;
+
+/// <summary>Short HEAD of a checkout, stamped into a finding's frontmatter so staleness is visible.</summary>
+public static class GitHead
+{
+    /// <summary>
+    /// Never throws — a fresh repo with no commits yet, or a working dir that is not a git repo
+    /// at all, must not block a finding save. Callers get "unknown" instead.
+    /// </summary>
+    public static async Task<string> ShortAsync(string workingDir, CancellationToken ct)
+    {
+        try
+        {
+            return await GitExcludeWriter.RevParseAsync(workingDir, ["rev-parse", "--short", "HEAD"], ct);
+        }
+        catch (InvalidOperationException)
+        {
+            return "unknown";
+        }
+    }
+}