Add jupyter notebook execution as CI test

[joezuntz]

Make sure the welcome notebook works

[Copilot]

Pull request overview

This PR adds a new CI job intended to execute the “Welcome to TXPipe” Jupyter notebook to ensure it continues to run successfully, while also making several small pipeline/IO fixes that support running the example workflow.

Changes:

• Add a GitHub Actions “Notebook” job that restores the cached conda env + example data and runs the welcome notebook.
• Fix/align Fourier/covariance metadata and map-handling (binning metadata parsing, mask/map Healpix reads, tracer bin counts).
• Update defaults/dependencies to support the example workflow (mask threshold default, bump sacc version).

Reviewed changes

Copilot reviewed 6 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
txpipe/utils/nmt_utils.py	Allow ell-binning recovery from SACC metadata by stripping binning/ prefixes.
txpipe/twopoint_fourier.py	Minor noise-metadata logic cleanup and a whitespace-only change.
txpipe/metadata.py	Add nbin_source / nbin_lens into produced metadata.
txpipe/maps.py	Change default mask_threshold for density-map generation.
txpipe/covariance.py	Switch to Healpix reads and resample mask to match density-map resolution; adjust mask-name mapping.
bin/environment-perlmutter.yml	Bump sacc[all] requirement.
bin/environment-local.yml	Bump sacc[all] requirement.
.github/workflows/ci.yml	Add Notebook CI job to execute the welcome notebook.

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