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.

docs/explore-notes/external-mcp.md

@@ -32,7 +32,14 @@ session internals, or app-settings writes. Auth via an optional `X-ClaudeDo-Key`
      `ListTasksResult`/`BatchGetTaskResult`, where exactly one of the lean (`TaskRefDto`) and
      full (`TaskDto`, incl. Description/Result) fields is populated per the flag — keeps a
      list of verbosely-described tasks from blowing past the response size limit by default.
-3. `ExternalMcpExceptionFilter.Wrap` is registered as a call-tool filter so
+3. **Description style is documented in `McpToolDocs`** (same folder) and shared boilerplate
+   lives there as `const` strings. Rules: the first sentence says what the tool does *and* when
+   to reach for it (MCP clients rank tools by that text, so the trigger must not sit behind
+   return-shape prose); parameters are documented with `[Description]` **on the parameter**, not
+   in the tool description; result fields appear only where the caller must branch on them
+   before calling (`isEmpty`, `truncated`, `conflicts`, `available`); no design rationale or
+   "since this feature was introduced" history. Not test-enforced — review it in PRs.
+4. `ExternalMcpExceptionFilter.Wrap` is registered as a call-tool filter so
    `InvalidOperationException` / `ArgumentException` messages survive as `McpException` —
    otherwise the SDK's catch-all replaces any non-`McpException` with a generic
    *"An error occurred invoking 'X'."*
@@ -42,8 +49,9 @@ session internals, or app-settings writes. Auth via an optional `X-ClaudeDo-Key`
 ### `ExternalMcpService` — task CRUD, execution, git
 
 Task: `ListTaskLists`, `ListTasks`, `GetTask`, `AddTask`, `AddSubtask`, `UpdateTask`,
-`UpdateTaskStatus`, `GetTaskStatusValues`, `ReviewTask`, `RunTaskNow`, `ContinueTask`,
-`CancelTask`, `DeleteTask`.
+`UpdateTaskStatus`, `ReviewTask`, `RunTaskNow`, `ContinueTask`, `CancelTask`, `DeleteTask`.
+(`GetTaskStatusValues` was removed — a whole tool entry for static reference text. `GetTask`'s
+description is now the canonical place for what each status means.)
 
 Worktree/git: `GetTaskWorktree`, `GetTaskDiff`, `MergeTask`, `ContinueMerge`, `AbortMerge`,
 `PreviewMerge`, `PreviewMergeSet`, `RevertMerge`, `ListWorktrees`, `CleanupTaskWorktree`.