Ship an optional format-on-edit hooks fragment and audit hooks in onboard

[prorochestvo]

Context

The kit standardizes permissions and marketplace wiring but ships zero hooks — the
strongest determinism lever in Claude Code. Formatting is left to the agent remembering
to run it, and pipeline:onboard does not audit hooks at all. The consuming projects
already lean on hooks elsewhere (CI notifications, relays), so the pattern is familiar;
it just isn't part of the scaffold.

Proposal (opt-in, not default)

• Add per-stack hooks fragments (e.g. a hooks/ dir or hooks-go.json /
  hooks-flutter.json alongside the permission fragments) with a PostToolUse hook that
  runs the formatter on the edited file: gofmt/goimports for Go, dart format for
  Flutter.
• Document them as opt-in in the README — do NOT force them into install.sh's
  default settings.json (auto-format-on-edit is opinionated and some will not want it).
• pipeline:onboard: add a check that reports whether hooks are wired and points to the
  fragment.

Acceptance criteria

• At least one documented, copy-pasteable format-on-edit hook per stack.
• README section explaining the trade-off and how to enable it.
• onboard mentions/audits hooks.
• Kept optional — default install.sh output is unchanged.