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). Centralised so the config loader, the CLI
(lean-ctx profile lean) and the dashboard all agree on the same set (#431).