mcpsrv: answer a real NULL as null

mcpsrv: name the cursor a commit-log miss was asked about

mcpsrv: a bad commit-log cursor is a miss, not a fault

mcpsrv: answer what is ready across every tracker

mcpsrv: list the memories a tracker holds

mcpsrv: add the beads tools

mcpsrv: add the generic browse tools

mcpsrv: serve a stateless read-only MCP surface