fix(sanitize): anchor sandbox-comment regex to avoid dropping real rows

[ddtcorex]

Summary

• sync --db and db import --stream-db were silently dropping rows because
  SanitizeSQLDump's sandbox-comment regex (.*999999.*sandbox.*) matched any
  dump line containing both substrings, not just the specific mysqlbinlog artifact
  comment (/*!999999\- enable the sandbox mode */) it was meant to strip.
• Since mysqldump --extended-insert (the default here) puts a whole table's rows
  on one line, real data containing "999999" (e.g. an ID) and "sandbox"
  (e.g. a PayPal/Braintree environment setting) anywhere on that line caused the
  entire INSERT statement to be dropped with no error.
• db import -f <file> doesn't sanitize file-based imports at all, which is why it
  imported full data while the streaming paths (sync --db, db import --stream-db)
  came up short — same root symptom, same root cause.
• Anchored sqlSandboxPattern to the real /*!999999 ... */ comment delimiters, and
  changed the fast-path substring gate from "sandbox" to the distinctive
  "/*!999999" marker.

Closes #109

Test plan

• Added regression test reproducing the false-positive drop on a synthetic
  --extended-insert-style line (tests/sql_sanitize_test.go)
• Updated the existing "Remove Sandbox lines" test to use the real comment
  format (/*!999999\- enable the sandbox mode */) and confirmed it's still
  dropped
• go build ./..., go vet ./..., go test ./... all pass
• gofmt -s -l . shows no drift on changed files