Replace the formatter dispatch with a generic {{placeholder}} lookup
system. Placeholders in prompt templates are resolved at runtime from
a table: topology, nodes, episodes, health, pairs, rename, split.
The query in the header selects what to operate on (keys for visit
tracking); placeholders pull in formatted context. Placeholders that
produce their own node selection (pairs, rename) contribute keys back.
Port health, separator, rename, and split agents to .agent files.
All 7 agents now use the config-driven path.
