The previous fix wired _resolve_user_default_mode() to look up
tools.session_search.default_mode, but the config schema has no
top-level 'tools' section. The closest analogue is auxiliary.<tool>,
which already groups per-tool config by tool name (auxiliary.vision
has download_timeout, auxiliary.session_search has max_concurrency —
neither is strictly aux-LLM routing).
This moves the lookup to auxiliary.session_search.default_mode so the
knob lives next to max_concurrency and the existing session_search
config block. Adds default_mode to the default config scaffold so it
shows up in fresh installs.
Updates docstring, tool description string, warning messages, and all
7 mock-config tests to the new path. 88/88 tests passing.
yoniebans
76f40e6449