mcpsrv: report the projection's clip on get_issue and list_milestones

Max clips every table read at 2000 rows. list_issues has always said so
(table_truncated, table_total); the other two answers computed over the
same read said nothing, so a rollup over a prefix of the tracker read as
arithmetic about the whole of it, and an id in the tail of a big tracker
was answered "no such issue" — a claim neither read can support.

get_issue now carries the two fields on every answer and splits the
miss: a complete read keeps the plain refusal, a clipped one says the id
was not among the rows read, names the cap and the true total, and ships
the payload (issue: null beside table_truncated) so the two misses are
one field apart rather than one adjective apart. list_milestones carries
the same two fields.

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