Update docs for agent-dispatch and agenttest paths

[Pummelchen]

Summary

Update docs for current agent-dispatch syscall and agenttest userspace path.

Problem / objective

Current source includes XAIOS_SYSCALL_AGENT_DISPATCH and runs /bin/agenttest; some docs and contracts appear to lag these paths.

Acceptance criteria

• Inspect current agent protocol source and userspace app behavior.
• Update docs/API and architecture docs to mention agent-dispatch and agenttest where appropriate.
• Keep contract changes coordinated with the syscall ABI issue.

Validation

• docs-only
• make qemu-abi-contract if contracts are changed

Relevant files

• kernel/runtime/agent_protocol.c
• kernel/include/xaios/agent_protocol.h
• kernel/user/syscall.c
• userspace/apps/agenttest.c
• docs/API.md
• docs/ARCHITECTURE.md