Auto-fetch LSN in stop-cdc.sh

[DmitriiAn]

stop-cdc.sh no longer takes an LSN argument. It queries
pg_current_wal_lsn() from the source, displays it, and asks
for confirmation [y/N] before writing to the sentinel — reducing
manual steps at cutover and adding a safety gate.

README and AGENTS.md updated to reflect the new usage

Unit tests for stop-cdc.sh

── environment validation ───────────────────────────────────────────────
  ok  missing env vars → exit 1
  ok  missing env vars → error names the variable

── psql LSN fetch ───────────────────────────────────────────────────────
  ok  psql failure → exit 1
  ok  psql failure → meaningful error
  ok  psql empty output → exit 1
  ok  psql empty output → meaningful error
  ok  fetched LSN is displayed before confirmation

── confirmation prompt ──────────────────────────────────────────────────
  ok  user answers n → exit 0
  ok  user answers n → aborted message
  ok  user presses Enter (empty) → exit 0
  ok  user presses Enter → aborted message
  ok  user answers Y → not aborted

── migration directory discovery ────────────────────────────────────────
  ok  no migration dir → exit 1
  ok  no migration dir → clear error
  ok  migration dir without source.db → exit 1
  ok  no source.db → clear error

── happy path ───────────────────────────────────────────────────────────
  ok  happy path → exit 0
  ok  happy path → confirmed sentinel message
  ok  happy path → LSN echoed in output
  ok  happy path → sentinel stored in DB
  ok  explicit MIGRATION_DIR → exit 0
  ok  explicit MIGRATION_DIR → sentinel confirmed
  ok  abort → sentinel not modified

All 23 tests passed.

[DmitriiAn]

@teknogeek0 once you approve it, I'll update this page as well: https://planetscale.com/docs/postgres/imports/postgres-migrate-pgcopydb#8-cutover