Split the query on whitespace into tokens; a learning matches if ANY
token appears as a substring in ANY of key/insight/files. Previously
the whole query was a single substring, so multi-word queries like
"debug investigation" only matched learnings whose insight contained
that exact contiguous phrase, which is usually nothing.
Whitespace-only query falls through to no-query (matches today's no-flag
behavior). Single-word queries behave exactly as before.
Adds test/gstack-learnings-search.test.ts: 3 assertions covering
multi-token, single-token, and no-query backwards compat.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
a0539b5f06