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 before they run.

version: '3'

preconditions:
  - sh: test -f .env
    msg: "Missing .env file. Run 'task init' to set up the project."

tasks:
  build:
    cmds:
      - go build ./...

• Global preconditions are checked before a task's own preconditions
• They are inherited by tasks in included Taskfiles (preconditions propagate downward through the include hierarchy)

Changes

• taskfile/ast/taskfile.go — add Preconditions field to Taskfile struct and UnmarshalYAML
• taskfile/ast/tasks.go — pass global preconditions through Tasks.Merge, prepending them to each task after deep copy
• taskfile/ast/graph.go — after full graph merge, prepend root taskfile's global preconditions to all tasks
• executor_test.go + test fixtures — 6 new test cases covering: precondition met, precondition not met, inheritance by included tasks, interaction with task-level preconditions
• website/src/docs/guide.md — new "Global preconditions" section
• website/src/docs/reference/schema.md — new preconditions entry in Root Schema
• website/src/public/schema.json — preconditions added to root taskfile properties