Remove HITL primitives

[nficano]

ARCP v1.1 explicitly delegates HITL to a companion protocol. PHP SDK has HITL artifacts that should be removed:

• samples/human_input/ — delete the sample directory
• README.md lines 126, 144, 147 — remove HITL rows from RFC mapping table
• samples/cancellation/README.md:28 — remove HITL relay doc
• src/Client/ARCPClient.php:62 — drop HITL/permission handler comment

Verify: grep -rEni 'HITL|human.in.the.loop|hitl_' src/ samples/ docs/ README.md returns zero hits.

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