fix(tools): clarify search_code path: vs filename: (#2343)

[beejak]

Summary

• Extend the \search_code\ query schema description so \path:\ (directory prefix) vs \ilename:\ (file name) is explicit.
• When GitHub returns zero code results and the query uses \path:\ with a file-like token (contains ., no path separators), append a second text content block pointing agents at \ilename:.

Context

Addresses the confusion described in #2343 (silent empty results when \path:\ is used like a filename filter).

Test plan

• \go test ./pkg/github -run 'Test_SearchCode|TestPathQualifier' -count=1\
• \UPDATE_TOOLSNAPS=true go test ./pkg/github -run '^Test_SearchCode$' -count=1\ (schema snap updated)