[jkrick]

In preparation for the upcoming DR1 updates. Closes #150, closes #238, closes #275.

Summary

• Switch TAP access from raw pyvo to astroquery.ipac.irsa.Irsa.query_tap
• Use SkyCoord instead of separate ra/dec floats
• writeto(..., overwrite=False) so output isn't silently clobbered
• Plot wavelength as an astropy Quantity instead of a bare float
• Drop a duplicated caching tip box in spherex_cutouts.md
• Preserve the cutout's original primary header (incl. VERSION) in the combined MEF's
  primary HDU instead of writing a blank one, and record each cutout's version in
  CUTOUT_INFO
• Fold the Source Discovery Tool README's directory-contents list into its notebook
  (the rest of the README was already duplicated there) and remove the README
• Open spherex_intro.md's spectral image with a context manager, reading all extensions into
  memory so the downstream cells work after the file is closed (spherex_intro.md part of Make all fits.open() calls use a context manager #129)
• Also treat http.client.RemoteDisconnected as a transient read error in spherex_cutouts.md
  (skip that cutout) and spherex_psf.md (retry), after CI failed on a server disconnect
  mid-read

Note on #129

spherex_cutouts.md, spherex_psf.md, and the SDT notebook already use context managers for
every fits.open() call. spherex_intro.md's bare fits.open() call is fixed here too (purely
mechanical, does not touch the QR3-stale content next to it, which stays for #374). The two
Euclid notebooks and the Roman notebook also listed in #129 are untouched -- out of scope for
this PR.

Test plan

• Ran spherex_cutouts.md and spherex_psf.md end to end against production
  (jupytext --to notebook --execute); no errors
• Verified VERSION is present in the output MEF's primary header
• Verified the SDT notebook still parses correctly after the README merge (full
  execution needs SExtractor/Firefly/bokeh, not run here)
• Ran spherex_intro.md end to end against production; no errors

[jkrick]

Added update to fix the failing notebook test. CI hit http.client.RemoteDisconnected while reading a SPHEREx
image, and that error wasn't in the list of transient errors we catch. It's now caught in both places that use
that list: spherex_cutouts.md skips the affected cutout, and spherex_psf.md's retry loop retries it (and still
raises after 3 attempts).