feat: add global preconditions at the taskfile level

[drichardson]

Closes #294

Summary

Adds a top-level preconditions key to Taskfiles that applies to all tasks in that file before they run.

version: '3'

preconditions:
  - sh: test -f .env
    msg: "Missing .env file. Run 'task init' to set up the project."
    inherit: true   # also applies to tasks in included Taskfiles

includes:
  tools: ./tools

tasks:
  init:
    skip_preconditions: true  # skips parent's inherited preconditions
    cmds:
      - cp .env.template .env

Behavior

• Global preconditions are file-scoped by default — they apply to tasks in the file they are defined in, but not to tasks in included Taskfiles
• Setting inherit: true on a precondition causes it to propagate into included Taskfiles, transitively through the include hierarchy
• A task with skip_preconditions: true opts out of inherited preconditions from parent Taskfiles; global preconditions from the task's own file still apply
• skip_preconditions has no effect on root-level tasks (they have no inherited preconditions)
• Global preconditions are checked before a task's own preconditions

Changes

Core:

• taskfile/ast/taskfile.go — Preconditions []*Precondition field on Taskfile
• taskfile/ast/precondition.go — Inherit bool field on Precondition
• taskfile/ast/task.go — SkipPreconditions bool field on Task
• taskfile/ast/tasks.go — scoped injection in Tasks.Merge: own-file tasks always get all preconditions; transitive-include tasks only get inherit: true preconditions unless skip_preconditions
• taskfile/ast/graph.go — same scoped logic applied to root preconditions after full merge

Tests (20 new cases):

• Global precondition met/not met for root and included tasks
• Non-inheriting precondition does not affect included tasks
• inherit: true blocks included tasks and propagates transitively (root → mid → deep)
• Mid-level inherit: true blocks sub-includes but does not affect root tasks
• skip_preconditions skips inherited preconditions
• skip_preconditions does not skip own-file preconditions
• skip_preconditions is a no-op on root tasks

Docs:

• website/src/docs/guide.md — new "Global preconditions" section with inherit and skip_preconditions usage
• website/src/docs/reference/schema.md — new entries in Root Schema, Task, and Precondition sections
• website/src/public/schema.json — preconditions on root taskfile, inherit on precondition object, skip_preconditions on task