feat(init): suggest JS/TS test-file excludes (#165) (#171)

Unexcluded co-located test files (*.spec.ts / *.test.ts / __tests__)
were the upstream root cause of graph-export edge pollution
(lh-enterprise: 77% of edges from test files, all mocks unresolved).
Fixed at the config seam — single source of truth for scan-all and
graph-export; a --exclude-tests flag was rejected as it would let the
two see different trees.

- infer_exclude_patterns: +7 test patterns, suggested only when such
  files exist (conditional style, same as vendor/target)
- config explain exclude: lists the patterns — graph-export's
  high-unresolved warning already told users to run it