fix(mcp): report a Failed task's failureReason instead of a bare status

get_task/batch_get_tasks now return failureReason (max_turns|timeout|error|
cancelled|unknown) plus failureTurnsUsed/failureMaxTurns on a Failed task, so
max_turns (worktree usually fine, continue_task) is distinguishable from a
real error (reset_failed_task) without pulling get_task_log's raw NDJSON.
Classified and stamped onto TaskEntity by TaskRunner.MarkFailed via
TaskStateService.FailAsync; TaskRunEntity also keeps the CLI's raw
terminal_reason/result_subtype/errors for deeper diagnosis. reset_failed_task's
description now warns explicitly that it discards the worktree and points at
continue_task for max_turns. Surfaced on the task card's status-chip tooltip.

src/ClaudeDo.Worker/External/BatchMcpTools.cs

@@ -42,7 +42,8 @@ public sealed class BatchMcpTools
     [McpServerTool, Description(
         "Fetch a snapshot of many tasks in one call — use for an overview or polling a fan-out instead of " +
         "calling get_task per id. A missing id comes back as found=false, not an error; error is only set " +
-        "for an unexpected failure." + McpToolDocs.MaxBatch)]
+        "for an unexpected failure. A Failed task's task/taskFull carries failureReason (see get_task)." +
+        McpToolDocs.MaxBatch)]
     public async Task<IReadOnlyList<BatchGetTaskResult>> BatchGetTasks(
         string[] taskIds,
         [Description("If true, return the full task (incl. Description/Result) in `taskFull`; if false " +

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -45,7 +45,13 @@ public sealed record TaskDto(
     // A planning/improvement child reporting > 0 still goes straight to Done (see
     // ClaudeDo.Worker/CLAUDE.md → Unified parent model) -- this is the only MCP-visible signal
     // that it may have delivered nothing despite that Done status.
-    int RoadblockCount = 0);
+    int RoadblockCount = 0,
+    // The three below are non-null only when Status=Failed; stamped by TaskRunner.MarkFailed via
+    // TaskStateService.FailAsync. failureReason is "unknown" for a Failed task that predates this
+    // field. Lets a caller triage without pulling get_task_log's raw NDJSON.
+    string? FailureReason = null,
+    int? FailureTurnsUsed = null,
+    int? FailureMaxTurns = null);
 
 // Lean counterpart to TaskDto for writing/status-changing tools: echoes back what changed
 // without re-sending Description/Result, which the caller just sent or already has.
@@ -56,7 +62,10 @@ public sealed record TaskRefDto(
     string Status,
     int SortOrder,
     bool IsMyDay,
-    int RoadblockCount = 0);
+    int RoadblockCount = 0,
+    string? FailureReason = null,
+    int? FailureTurnsUsed = null,
+    int? FailureMaxTurns = null);
 
 // tasks is populated when includeDescription=false (the default): lean references, no
 // Description/Result. tasksFull is populated when includeDescription=true: full tasks incl.
@@ -211,7 +220,8 @@ public sealed class ExternalMcpService
         "A successful run lands in WaitingForReview; use review_task to approve, reject or cancel it. " +
         "Done/Failed/Cancelled tasks can be reset to Idle for re-execution. A Queued task with a blocker waits " +
         "for its predecessor before the picker will claim it, and WaitingForChildren is a parent whose own work " +
-        "is done but whose children are still running.")]
+        "is done but whose children are still running. For Status=Failed, failureReason (max_turns|timeout|" +
+        "error|cancelled|unknown) plus failureTurnsUsed/failureMaxTurns say why without pulling get_task_log.")]
     public async Task<TaskDto> GetTask(string taskId, CancellationToken cancellationToken)
     {
         var task = await _tasks.GetByIdAsync(taskId, cancellationToken)
@@ -1326,7 +1336,10 @@ public sealed class ExternalMcpService
         t.FinishedAt,
         t.IsMyDay,
         t.SortOrder,
-        t.RoadblockCount);
+        t.RoadblockCount,
+        FailureReasonOf(t),
+        t.Status == TaskStatus.Failed ? t.FailureTurnsUsed : null,
+        t.Status == TaskStatus.Failed ? t.FailureMaxTurns : null);
 
     private static TaskRefDto ToRefDto(TaskEntity t) => new(
         t.Id,
@@ -1335,7 +1348,15 @@ public sealed class ExternalMcpService
         t.Status.ToString(),
         t.SortOrder,
         t.IsMyDay,
-        t.RoadblockCount);
+        t.RoadblockCount,
+        FailureReasonOf(t),
+        t.Status == TaskStatus.Failed ? t.FailureTurnsUsed : null,
+        t.Status == TaskStatus.Failed ? t.FailureMaxTurns : null);
+
+    // "unknown" covers a Failed task that predates this field (never got a classified reason
+    // stamped) — a defined value rather than null so callers don't have to special-case it.
+    private static string? FailureReasonOf(TaskEntity t) =>
+        t.Status == TaskStatus.Failed ? (t.FailureReason ?? "unknown") : null;
 }
 
 internal static class DailyPrepFilter

src/ClaudeDo.Worker/External/LifecycleMcpTools.cs

@@ -20,7 +20,12 @@ public sealed class LifecycleMcpTools
         _reset = reset;
     }
 
-    [McpServerTool, Description("Reset a failed task back to Idle so it can be run again, discarding its now-stale worktree. Only tasks with Status=Failed are accepted; other statuses throw.")]
+    [McpServerTool, Description(
+        "Discards a failed task's worktree (and all uncommitted work in it) and resets it to Idle for a fresh " +
+        "run. Only tasks with Status=Failed are accepted; other statuses throw. Check failureReason from " +
+        "get_task/batch_get_tasks first: for failureReason=\"max_turns\" the worktree's work is usually still " +
+        "good and continue_task (resume the session, keep the worktree) is almost always the right call instead " +
+        "— reach for this tool only for a real error, not a task that just ran out of turns.")]
     public async Task<ResetFailedTaskResult> ResetFailedTask(string taskId, CancellationToken cancellationToken)
     {
         var task = await _tasks.GetByIdAsync(taskId, cancellationToken)