Remove HITL primitives

[nficano]

HITL is out of scope per ARCP v1.1. Remove:

• src/messages/human.rs — delete file; drop Human* variants from message enum in src/messages/mod.rs
• tests/human_input.rs — delete
• examples/human_input/ — delete entire directory
• src/lib.rs:12 — drop "human-in-the-loop" reference

Verify: grep -rEni 'human|hitl' src/ tests/ examples/ returns no non-trivial hits.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)