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 " +