./stack init: create .env-backup and .env-notify when asked #3

./stack init creates the machine's .env and every enabled stack's .env from their examples. Backups and alerts are optional, so it only prints optional, not created: .env-backup (cp .env-backup.example .env-backup && chmod 600 .env-backup) — one more manual step on every new machine that wants them.

What to build: ./stack init --with backup,notify (or a similar flag that cannot be mistaken for a stack name) also creates those two files from their examples, mode 600, never overwriting an existing one, and names their CHANGE_ME values like the others.

Where to look:

• platform/bin/stack.sh, verb_init and init_create.
• platform/bin/selftest.sh, block "== stack init".

Done when: a selftest check creates both on request, leaves them alone without the flag, and init fails while they still hold CHANGE_ME.