Generate JSON sidecars for BIDS derivative outputs

[CMonnin]

Follow-up from the BIDS-derivatives review on #11 (deferred to a separate PR).

The pipeline currently emits zero JSON sidecars. A BIDS-compliant derivative dataset needs metadata alongside each output.

Required / recommended fields per output

Output	Sidecar	Fields
..._T1w.mnc (main)	..._T1w.json	SkullStripped (REQUIRED, bool), Sources, Description
..._desc-denoised_T1w.mnc	.json	SkullStripped, Sources, Description
..._label-brainwithcsf_mask.mnc	.json	Type: "Brain" (REC), Sources
..._label-brainnocsf_mask.mnc	.json	Type: "Brain", Sources
..._dseg.mnc	.json	SpatialReference (REC; REQUIRED if not in a standard space), Sources
_space-LSQ6_* variants	.json each	same as non-LSQ6 counterpart

Open questions

• SkullStripped value: the main _T1w output is the bias-corrected whole-head volume (synthstrip_N3.sh:913-915), masks emitted separately -> false. Confirm intended.
• Whether to also emit a dataset-level dataset_description.json (GeneratedBy, DatasetType: "derivative") -- currently files drop next to the input rather than into a derivatives tree, so arguably out of scope.

Spec

• https://bids-specification.readthedocs.io/en/stable/derivatives/imaging.html