[6.x] Sidecar

[jackmcdade]

Summary

Experimental: Sidecar — a core framework for editing external markdown content (docs sites, SSGs) as Statamic collections in the Control Panel, in place, without importing or changing how the source system builds.

This PR ships the CMS portion only. Concrete adapters live in separate packages (statamic/sidecar-laradocs, statamic/sidecar-jigsaw, …).

Why

Teams already have content in LaraDocs, Jigsaw, etc. Sidecar lets them keep that filesystem contract and still get Statamic’s CP: blueprints, structure trees, Live Preview, permissions — pointed at the real files.

What’s in core

• Sidecar manager + driver contract — packages register via Sidecar::extend() / Sidecar::pair(); collections are declared in config/statamic/sidecar.php
• In-memory collection registration — Collection::register() so Sidecar collections don’t need a content/collections/*.yaml file (on-disk cascade data is still preserved when present)
• Custom entry directories — collections can resolve entries from outside content/collections/{handle}
• Nested-folder sync — drivers that opt in (usesNestedFolders()) rewrite paths from the structure tree (page.md ↔ page/_index.md, prune empty dirs, sync sibling order)
• Install UX — php please sidecar:install detects paired SSG packages and scaffolds config
• Live Preview without routes — Collection::hasLivePreview() + driver previewUrl() so headless/Sidecar collections can still Visit / preview
• Blueprint fallbacks — collections can supply a driver blueprint when none exist on disk
• Removes path-slug experiment — nesting is real folders (or driver-owned nav), not slash-containing slugs

Driver packages (out of this PR)

Package	Model
statamic/sidecar-laradocs	Nested folders + _index.md + order FM
statamic/sidecar-jigsaw	Flat source/docs/ + navigation.php as hierarchy

Example

// config/statamic/sidecar.php
'collections' => [
    'docs' => [
        'driver' => 'laradocs',
        'directory' => base_path('docs'),
    ],
],

composer require petebishwhip/laradocs statamic/sidecar-laradocs
php please sidecar:install laradocs

• php please sidecar:install with no drivers / one detected package / multiple packages
• Sidecar collection appears in CP without a collection YAML
• Entries load from a custom directory; save preserves unknown front matter
• Nested-folder driver: reorder/nest/unnest in Tree view relocates files, writes order, deletes empty dirs
• Deleting a Sidecar collection does not wipe the external content directory
• Live Preview / Visit URL works with previewTargets / driver previewUrl and no collection route
• Multi-site: core still requires site folders; custom directories may store flat entries
• Existing non-Sidecar collections/stache behavior unchanged
• Run tests/Sidecar/*, tests/Data/Entries/CollectionTest, EntriesStoreTest, CollectionRepositoryTest