refactor(services): clean the service drivers and their comments

[ttncode]

What this changes

PR 3 of 5 (spec: docs/superpowers/specs/2026-09-16-clean-shell-and-comments-design.md). Covers services/**/*.sh.

• Headers: each of the 15 files now opens with a one-line description; comments are cut down to why-only.
• Tests: the remaining shell [ ] tests in the drivers are now [[ ]].
• Helpers: the probe splice moved out of the longest service_driver_apply bodies into same-file helpers splice_nest_probe, splice_laravel_probe and splice_mongodb_probe, the same shape as the existing splice_flask_probe. Driver interface functions are unchanged.
• Unchanged: the logic of assert_nest_probe_spliced and the flask splice guard, plus every byte written into generated projects.

How it was verified

• Lint: mise run lint exits 0.
• Tests:
  • tests/service.bats 48/48.
  • test-integration 69/69.
  • test-unit 195/196. The one failure is install.bats "start_stack logs in to ghcr only when a token is set". It fails the same way on main and exercises common/install.sh, which this PR does not touch. Its stub docker never reads stdin, so the printf feeding --password-stdin can hit SIGPIPE.
• Generated projects:
  • mise run checklist exits 0 for both --api flask --web nextjs --db postgres --cache redis and --api nestjs --db mongodb.
  • Both generated trees were diffed against the same projects generated from main and are byte-identical, apart from the -dirty git-describe suffix.

Checklist

• mise run lint passes
• mise run test-runner passes — see the pre-existing install.bats flake above
• New behaviour has a test that fails without the change
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes