Make mise the dev entrypoint for the headless e2e suite, matching the repo's
mise-tasks convention (auto-discovered, shows in `mise tasks`). Dev relies on
system-installed nvim + rustc; CI will provide them via Dagger (slice 11c).
The self-contained shim runner is unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
