fix(mcp): keep ADR guidance aligned with schema

[zgqq]

Summary

• replace the invalid manage_adr(mode='store') hint with the accepted update mode
• source the schema enum and all guidance from one constant to prevent drift
• add a tool-schema regression assertion

Validation

• make -f Makefile.cbm build/c/test-runner
• MCP tool-list regression reaches PASS mcp_tools_list under ASAN with leak detection disabled

Existing suite constraints

• the full local suite also reports pre-existing incremental RSS > 2 GiB and a cbm_mcp_tools_list LeakSanitizer allocation; neither path is changed by this patch.