refactor(runner): move `trail tune` to `entire runner setup`

Reframe the tuning command as setup of a repo's runners, under a new runner group (hidden during maturation, like trail):

• entire trail tune → entire runner setup.
• Renamed trail_tune_.go → runner_.go and the command type/funcs (newRunnerSetupCmd, runRunnerSetup, runnerSetupOptions, runnerInsecureHTTP).
• On re-run in a repo that already has runners (nothing to scaffold), setup now offers to re-tune instead of silently re-emitting: interactive confirm, or --run as an explicit yes; non-interactive without --run prints a hint.

First-run behavior (create defaults → tailor) and all flags (--run, --yes, --sources, --limit, --debug-dir, [runner]) are unchanged.