Intent Phase 4 follow-up: fix exec_cmd code type (Nat→Int)

Fix exec_cmd return code typing and align docs/tests.

Current state:

• Builtin type models exec_cmd result field code as Nat.
• Real subprocess exit codes can be negative (signals), so Int is more accurate.

Required behavior: 1. Change exec_cmd result code field type from Nat to Int in builtin environment. 2. Align docs in LANG.md (IO operations/typing examples) with runtime reality. 3. Add/adjust tests covering type expectations for exec_cmd result structure.

Optional:

• If convenient, include a small runtime test that demonstrates negative return code handling path.

Acceptance:

• Type model no longer constrains exit code to Nat.
• Docs and tests match the implemented runtime semantics.