Define task/target execution engine

[DamianReeves]

Background:

• Targets should orchestrate pipelines and tasks (build/test/clean).

Scope:

• Implement target definitions and task execution primitives.
• Support intrinsic actions and external command execution.
• Define configuration shape in morphir config.

Acceptance criteria:

• Targets can orchestrate pipelines/tasks.
• External commands run with explicit mount permissions.