SDSS: query sky-survey records and retrieve spectra

# SDSS Use Python 3.11 or later and HTTPS access to `skyserver.sdss.org`, the SDSS Science Archive Server and `api.sdss.org` for Valis delivery. The helper uses only the Python standard library; no credentials or website are required for these public interfaces. Resolve `scripts/sdss.py` relative to this installed skill folder, regardless of the working directory. On Windows, `py -3.11` can replace `python` in the examples. Establish ICRS RA/Dec in decimal degrees and radius in arcminutes, or an exact identifier with its namespace. If given a name, resolve and verify its coordinates with an authoritative astronomy resolver first; this helper does not resolve names. Read [references/data-access.md](references/data-access.md) for source contracts, supported parameter joins and bounds. Run `python scripts/sdss.py cone --ra 229.525575753922 --dec 42.7458537608544 --radius 2` for DR20 ALLSPEC records, or `target --id-type sdss_id --id ID` for a target's records. `allspec_id` is an alphanumeric record identifier; `sdss_id` identifies a cross-matched target; `specobjid` identifies a pipeline spectrum. Preserve every identifier as a string. A target may have several observations and products. Results are ordered by ID, not distance. If `has_more` is true, repeat the identical query with `--after NEXT_AFTER`; retain all pages and report any stopping cap. Empty results mean no returned records in that query, not an empty sky or absent archive data. Use `photo-cone` for DR17 primary imaging measurements in the r band, or `photo-id --id OBJID` for an exact photometric record (retain its mode). The photometric `specobjid` is an exact legacy spectrum association when present; do not attach a nearby DR20 spectrum as though it were that association. Point-like/extended morphology is not confirmed STAR/GALAXY. Do not derive photometry or object class from RGB imagery. DR20 has no new SDSS imaging; any SDSS DR9 color HiPS background remains DR9. Read [references/interpretation.md](references/interpretation.md) before reporting measurements. Preserve class, subclass, z, z_err, zwarning, instrument, run2d, coadd and both catalog/archive provenance. Missing fields, empty strings and survey sentinel values are unavailable measurements. `pipeline_redshift_unflagged` means only a finite z and zero warning; it is not a guarantee. SDSS recommends its visually inspected BHM catalog for robust classification/redshift work; this helper does not load that catalog. Do not turn stellar z into cosmological distance or calculate lookback time without explicit cosmology and justified redshift quality. Retrieve one exact record with `spectrum --id ALLSPEC_ID --output spectrum.fits > spectrum.json`. For DR20 BOSS v6_2_1 daily lite products, add `--delivery valis` to use the official file service; it first verifies that Valis resolves the exact catalog archive path. Other products use the default `--delivery sas`. Inspect available records before choosing an observation/product; do not assume every imaged target has a spectrum. Downloads preserve original FITS bytes and report catalog/archive provenance, delivery URL, full record and SHA-256. A missing or unsupported URL, service failure or size limit must be reported as such; no automatic fallback or insecure transport is used. This helper does not decode every instrument; use the file's official data model and a maintained FITS reader for analysis. Optical units, masks and uncertainty caveats are in [references/interpretation.md](references/interpretation.md). See [references/example.md](references/example.md) for a real region, exact legacy association and a new DR20 spectrum. Retain machine-readable output alongside analyses. Check [NOTICE.md](NOTICE.md) for authorship and survey attribution.