Previously buildInstructions emitted one line per collection with name,
doc count, and description, which can run to ~1.5 KB for a dozen
collections and is injected into every session's system prompt at
MCP initialize. Most agents never query qmd in a given session, so the
catalogue lines are a recurring token cost for static info that the
existing 'status' tool already exposes on demand.
Now emit a single comma-joined names line plus a hint that 'status'
returns the rest. listContexts() lookup is dropped since the per-
collection descriptions are no longer rendered.
Refs #647
9cecdc8703