Adds a top-level `preconditions` key to Taskfiles that applies to all
tasks. Preconditions from a parent taskfile are inherited by tasks in
included taskfiles (propagated downward through the include hierarchy).

Closes go-task#294

Assisted by AI

Co-Authored-By: Claude <noreply@anthropic.com>

Extends global preconditions with two new fields:

- `inherit: bool` on a precondition: when true, the precondition
  propagates to tasks in included Taskfiles (transitively). Defaults
  to false, so preconditions are file-scoped by default.

- `skip_preconditions: bool` on a task: when true, the task does not
  receive preconditions inherited from parent Taskfiles. Global
  preconditions defined in the same file as the task still apply.
  Has no effect on root-level tasks.

Assisted by AI

Co-Authored-By: Claude <noreply@anthropic.com>