feat(run): define runtime standard-input policies

[GaspardKirira]

Summary

Make standard-input behavior explicit for interactive, captured, detached, and
watch executions.

Proposed policies

• inherit;
• pipe;
• closed;
• pseudo-terminal.

Requirements

• Select a default policy by execution mode.
• Preserve interactive command behavior.
• Prevent background processes from reading unintended input.
• Support tests with piped input.
• Expose the resolved policy in run plans.

Acceptance criteria

• Interactive programs can inherit input.
• Tests can provide piped input.
• Detached processes can close input.
• PTY execution is represented explicitly.
• Invalid policy combinations return diagnostics.