fix(import): clean up local files and _subfiles dirs for remotely-deleted workflows

[jspaans91]

Summary

• --cleanup-orphans was documented as deleting "local files without matching remote workflow", but only ever handled files with no embedded workflow ID. A local file whose workflow had since been deleted in n8n (i.e. it has an ID, but that ID no longer exists remotely) was never touched. Now every remote workflow ID seen during the sync pass is tracked, and any local file whose embedded ID isn't in that set gets deleted (respecting --ids scoping, and leaving archived/tag-filtered-out workflows alone since they still exist remotely).
• --cleanup-subfiles now independently removes any _subfiles/ directory whose encoded workflow ID has no matching local workflow file — whether that's because --cleanup-orphans just deleted it, or the file was already gone (manual deletion, or an older n8n-cli that didn't clean up after itself). This is gated purely on --cleanup-subfiles, not on --cleanup-orphans, matching how the flag is documented.
• Dry-run mode reports both kinds of cleanup without touching disk.

Test plan

• Added tests/importer/cleanup-orphans.test.ts covering: stale file deletion, keeping files that still exist remotely, keeping archived (not deleted) workflows, dry-run reporting only, --ids scoping, _subfiles directory cleanup (both tied to a just-deleted workflow and pre-existing orphaned directories with no local file at all), and that _subfiles cleanup requires --cleanup-subfiles independently of --cleanup-orphans.
• bun test — full suite passes (1997 tests)
• bunx tsc --noEmit — clean
• bun run lint — clean on changed files