Removes the tool_profile key from config.toml, restoring the lean
default: only the lazy core set is advertised in tools/list,
while every registered tool stays reachable through ctx_call. This is
the recommended low-overhead mode (#575).
""/lean/lazy/reset are not pinned tiers — they are the unpin
sentinel that clears any pin so the default returns (lazy core advertised,
everything callable via ctx_call). Empty/whitespace is the documented
default of tool_profile (#613); treating it as unpinned avoids a spurious
Unknown tool_profile '' warning on the documented default. Centralised so
the config loader, the CLI (lean-ctx profile lean) and the dashboard all
agree on the same set (#431).