Wire up prettier-plugin-sh for shell files in lint-staged/CI

[tomgrv]

Problem

prettier-plugin-sh is already an installed dependency in the root package.json, but *.sh files are absent from the lint-staged glob, so the plugin is paid for (install size, config) but never actually applied to any shell script in the repo.

Proposal

• Add *.sh (and any other shell-script extensions used, e.g. files without extension under src/*/) to the lint-staged configuration in root package.json so prettier with prettier-plugin-sh formats them on commit.
• Run a one-time repo-wide format pass once the config is in place so existing scripts aren't all flagged as diffs on next touch.
• Cross-check formatting rules against ShellCheck (see the separate "Add ShellCheck linting to CI" issue) to make sure the two tools don't fight each other (e.g. brace/quoting style).

Where this lives / sync note

This is a root-level tooling config change (package.json, .prettierrc/lint-staged config) — it governs the monorepo's own pre-commit hooks (which this repo dogfoods via its own githooks feature). It doesn't need to propagate into the split repos itself, but any shell files it reformats inside src/<feature>/ will flow through split-monorepo.yml as normal.

Acceptance criteria

• *.sh files are covered by lint-staged/prettier on commit.
• A full-repo formatting pass is committed once the config lands, with no further unrelated diff noise.
• No conflicts with ShellCheck's expectations.