refactor(worker): extract OverrideSlotService and reorganize Worker/Services into domain folders

Slice 5 of the worker state consolidation refactor.

OverrideSlotService (new in Worker/Queue/) owns RunNow, ContinueTask,
and the override-slot piece of CancelTask. QueueService keeps the
queue-slot guard for "task is already running" rejection and delegates
to OverrideSlotService for execution; CancelTask tries the override
slot first, then the queue slot. QueueSlotState is extracted to its own
file.

Folder reorg (via git mv to preserve history):
- Worker/Queue/      QueueService, OverrideSlotService, QueueSlotState
                     (alongside existing waker/picker)
- Worker/Lifecycle/  StaleTaskRecovery, TaskResetService, TaskMergeService
- Worker/Worktrees/  WorktreeMaintenanceService
- Worker/Agents/     AgentFileService, DefaultAgentSeeder

Worker/Services/ folder removed. All consumers updated to the new
namespaces (Program.cs, WorkerHub, ExternalMcpService,
PlanningMergeOrchestrator, all Worker tests).

OverrideSlotService is registered as a DI singleton in both the main
worker app and the external MCP app.

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

src/ClaudeDo.Worker/Agents/AgentFileService.cs

@@ -0,0 +1,76 @@
+using ClaudeDo.Data.Models;
+
+namespace ClaudeDo.Worker.Agents;
+
+public sealed class AgentFileService
+{
+    private readonly string _agentsDir;
+
+    public AgentFileService(string agentsDir)
+    {
+        _agentsDir = agentsDir;
+    }
+
+    public Task<List<AgentInfo>> ScanAsync(CancellationToken ct = default)
+    {
+        var agents = new List<AgentInfo>();
+        if (!Directory.Exists(_agentsDir))
+            return Task.FromResult(agents);
+
+        foreach (var file in Directory.EnumerateFiles(_agentsDir, "*.md"))
+        {
+            ct.ThrowIfCancellationRequested();
+            var (name, description) = ParseFrontmatter(file);
+            agents.Add(new AgentInfo(name, description, file));
+        }
+
+        agents.Sort((a, b) => string.Compare(a.Name, b.Name, StringComparison.OrdinalIgnoreCase));
+        return Task.FromResult(agents);
+    }
+
+    public async Task<string> ReadAsync(string path, CancellationToken ct = default)
+    {
+        return await File.ReadAllTextAsync(path, ct);
+    }
+
+    public async Task WriteAsync(string path, string content, CancellationToken ct = default)
+    {
+        var dir = Path.GetDirectoryName(path);
+        if (dir is not null) Directory.CreateDirectory(dir);
+        await File.WriteAllTextAsync(path, content, ct);
+    }
+
+    public Task DeleteAsync(string path, CancellationToken ct = default)
+    {
+        ct.ThrowIfCancellationRequested();
+        if (File.Exists(path)) File.Delete(path);
+        return Task.CompletedTask;
+    }
+
+    private static (string name, string description) ParseFrontmatter(string filePath)
+    {
+        var fileName = Path.GetFileNameWithoutExtension(filePath);
+        string name = fileName;
+        string description = "";
+
+        try
+        {
+            using var reader = new StreamReader(filePath);
+            var firstLine = reader.ReadLine();
+            if (firstLine?.Trim() != "---")
+                return (name, description);
+
+            while (reader.ReadLine() is { } line)
+            {
+                if (line.Trim() == "---") break;
+                if (line.StartsWith("name:"))
+                    name = line["name:".Length..].Trim();
+                else if (line.StartsWith("description:"))
+                    description = line["description:".Length..].Trim();
+            }
+        }
+        catch { /* Can't read file -- use filename fallback */ }
+
+        return (name, description);
+    }
+}