feat(worker): build/test the merge preview, not just the post-merge merge

preview_merge could only see file-overlap cleanliness via git merge-tree, never
whether the result compiles -- the two costliest findings of the 2026-08-06 batch
run were both merge-tree-clean but build-broken. When a list has a verify command
configured, a clean preview is now additionally materialized (via a commit-tree +
detached scratch worktree, outside the real repo, always cleaned up) and
built/tested there, without ever touching the real working tree.

preview_merge always attempts a verify run when a command is configured;
preview_merge_set only does when its new runVerify parameter is set (default off),
so a set preview never starts N builds unasked. The post-merge verify gate is
unchanged.

src/ClaudeDo.Data/Git/GitService.cs

@@ -3,7 +3,10 @@ using System.Text;
 
 namespace ClaudeDo.Data.Git;
 
-public sealed record MergePreview(bool Supported, bool Clean, IReadOnlyList<string> ConflictFiles);
+// TreeOid is only populated when Clean — the tree `merge-tree --write-tree` would produce,
+// usable to materialize the merge result into a scratch worktree without touching the real
+// working tree, index, or refs (see GitService.CommitTreeAsync / WorktreeAddDetachedAsync).
+public sealed record MergePreview(bool Supported, bool Clean, IReadOnlyList<string> ConflictFiles, string? TreeOid = null);
 
 public sealed class GitService
 {
@@ -357,7 +360,8 @@ public sealed class GitService
             ["merge-tree", "--write-tree", "--name-only", targetBranch, sourceBranch], ct);
 
         if (exitCode == 0)
-            return new MergePreview(true, true, Array.Empty<string>());
+            // stdout is just the written tree's oid on a clean merge.
+            return new MergePreview(true, true, Array.Empty<string>(), stdout.Trim());
 
         if (exitCode == 1)
         {
@@ -377,6 +381,62 @@ public sealed class GitService
         return new MergePreview(false, false, Array.Empty<string>());
     }
 
+    /// <summary>Resolves <paramref name="revision"/> (a branch, tag, or SHA) to a full commit SHA.</summary>
+    public async Task<string> RevParseAsync(string repoDir, string revision, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, stderr) = await RunGitAsync(repoDir, ["rev-parse", revision], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git rev-parse '{revision}' failed (exit {exitCode}): {stderr}");
+        return stdout.Trim();
+    }
+
+    /// <summary>
+    /// Creates a new commit object wrapping <paramref name="treeOid"/> with <paramref name="parentSha"/>
+    /// as its single parent, writing only a loose object — no ref is created or moved.
+    /// </summary>
+    public async Task<string> CommitTreeAsync(
+        string repoDir, string treeOid, string parentSha, string message, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,
+            ["commit-tree", treeOid, "-p", parentSha, "-m", message], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git commit-tree failed (exit {exitCode}): {stderr}");
+        return stdout.Trim();
+    }
+
+    /// <summary>
+    /// Materializes <paramref name="commitish"/> into a new, branchless worktree at
+    /// <paramref name="worktreePath"/> (detached HEAD) — used to build/verify a merge-tree
+    /// result without ever creating a branch or touching the real working tree.
+    /// </summary>
+    public async Task WorktreeAddDetachedAsync(
+        string repoDir, string worktreePath, string commitish, CancellationToken ct = default)
+    {
+        await WorktreeAddGate.WaitAsync(ct);
+        try
+        {
+            const int maxAttempts = 3;
+            for (var attempt = 1; ; attempt++)
+            {
+                var (exitCode, _, stderr) = await RunGitAsync(repoDir,
+                    ["worktree", "add", "--detach", worktreePath, commitish], ct);
+                if (exitCode == 0)
+                    return;
+
+                var transient = stderr.Contains("commondir", StringComparison.OrdinalIgnoreCase)
+                    || stderr.Contains("failed to read", StringComparison.OrdinalIgnoreCase);
+                if (!transient || attempt >= maxAttempts)
+                    throw new InvalidOperationException($"git worktree add --detach failed (exit {exitCode}): {stderr}");
+
+                await Task.Delay(150 * attempt, ct);
+            }
+        }
+        finally
+        {
+            WorktreeAddGate.Release();
+        }
+    }
+
     /// <summary>Count of files that differ on <paramref name="sourceBranch"/> since its merge base with the target.</summary>
     public async Task<int> CountChangedFilesAsync(
         string repoDir, string targetBranch, string sourceBranch, CancellationToken ct = default)