refactor(mcp): rewrite external MCP tool descriptions for trigger clarity

Every tool description now leads with what the tool does AND when to reach for
it, since MCP clients rank tools by that text. Per-parameter prose moved onto
the parameters as [Description], exhaustive result-shape enumerations and
design/history rationale dropped, and the repeated boilerplate clauses
(lean-task-ref, batch cap, refused-while-Running) pulled into McpToolDocs,
which also documents the style for future tools.

Tool-level description text: 20494 -> 13605 chars (-34%); combined with the new
parameter descriptions 18517 (-10%).

Closes gaps that caused wrong calls rather than just verbose ones:
- list_task_attachments returns metadata only, no file content
- run_task_now shares continue_task's single override slot and throws when busy
- list_runs is ordered oldest-first and feeds get_run
- workingDir on create_list/update_list is an existing local git repo path,
  unvalidated until the first task run
- get_task_worktree's behind=0 also means the main ref was unreachable

Removes get_task_status_values: a whole tool entry for static reference text.
GetTask's description is now the canonical place for status meanings.

src/ClaudeDo.Worker/External/HandoffMcpTools.cs

@@ -20,13 +20,15 @@ public sealed class HandoffMcpTools
     }
 
     [McpServerTool, Description(
-        "End of Phase 2 for the list handler (\"Let Claude handle it\"): hand this run off to a fresh " +
-        "ConPTY session that carries out Phases 3-5, without dragging along this session's dedupe/rewrite " +
-        "context. taskId is this session's own handler task id; survivingTaskIds are the tasks that made " +
-        "it past dedupe, in the order to run them. Reuses the SAME handler task -- no new task is created, " +
-        "and HandlerBaseCommit is untouched. The current tile stays open; end your own turn after calling this.")]
+        "Call at the end of Phase 2 of the list handler (\"Let Claude handle it\") to hand this run off " +
+        "to a fresh ConPTY session that carries out Phases 3-5, without dragging along this session's " +
+        "dedupe/rewrite context. Reuses the SAME handler task -- no new task is created, and " +
+        "HandlerBaseCommit is untouched. The current tile stays open; you must end your own turn " +
+        "immediately after calling this.")]
     public async Task<HandoffListHandlerResult> HandoffListHandler(
-        string taskId, IReadOnlyList<string> survivingTaskIds, CancellationToken cancellationToken)
+        [Description("This session's own handler task id.")] string taskId,
+        [Description("The tasks that made it past dedupe, in the order to run them.")] IReadOnlyList<string> survivingTaskIds,
+        CancellationToken cancellationToken)
     {
         if (survivingTaskIds.Count == 0)
             throw new InvalidOperationException("survivingTaskIds must contain at least one task id.");