Add lifecycle hooks for tasks and the overall workflow graph

[Copilot]

Implements onSuccess and onFailure lifecycle hooks at two scopes: individual tasks and the overall workflow graph.

New types (internal/types/lifecycle.go)

• LifecycleHook — specifies a hook command via command (array) or sh (inline script), matching the existing task command pattern
• Lifecycle — groups onSuccess and onFailure hooks; nil-safe accessors on both Lifecycle and Task

Per-task hooks (Task.Lifecycle)

Run after a task transitions to succeeded or failed, before any restart logic:

tasks:
  build:
    command: [go, build, .]
    lifecycle:
      onSuccess:
        sh: echo build succeeded | notify-slack
      onFailure:
        command: [./scripts/alert.sh, build failed]

Graph-level hooks (Spec.Lifecycle)

Run when the entire workflow exits. onSuccess fires only on clean internal completion (not on Ctrl+C); onFailure fires when any task fails without restart. Graph hooks use context.Background() since the main context is already cancelled at that point.

lifecycle:
  onSuccess:
    sh: echo all tasks completed
  onFailure:
    sh: ./scripts/send-alert.sh
tasks:
  ...

All hooks are best-effort — failures are logged but do not affect task or graph outcome. JSON schema updated to reflect new types.

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