Replace master references with main after default branch rename

[brendancol]

Summary

• Removes master from the CI push-trigger bridge added in Allow CI to trigger on main branch #1255 — test.yml and benchmarks.yml now trigger only on main.
• Updates the ASV benchmark comparison baseline from origin/master to origin/main and adjusts the step label.
• Sets benchmarks/asv.conf.json branches list to ["main"].
• Updates README.md (license link, Codecov badge) and CONTRIBUTING.md prose.
• Updates docs/source/index.rst (10 example URLs), docs/source/getting_started/usage.rst, and benchmarks/README.md.
• Updates .claude/commands/ release, bench, validate, and efficiency-audit scripts.

Context

Second step of the master → main default branch rename. The first PR (#1255) widened CI triggers to accept both names so there was no coverage gap across the GitHub-side rename. This PR drops the bridge now that main is the default.

A few "master" occurrences are intentionally left alone:

• actions/checkout@master and actions/setup-python@master in .github/workflows/test.yml — these refer to the master branches of the actions' own repos, not this one. Unpinned @master is a separate hygiene concern worth a follow-up PR to pin to @v4/@v5.
• master_doc references in docs/source/conf.py and Sphinx templates — this is Sphinx's own config variable name.
• The ASV default-branch comment in asv.conf.json — documents ASV's upstream behavior, not this project's branch.

Contributor migration

If you have a local clone of this repository, run:

```bash
git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a
git remote prune origin
```

Test plan

• CI passes on this PR (pull_request trigger fires as before).
• After merge, a push to `main` triggers both `pytest` and `Benchmarks` workflows.
• The Codecov badge on `README.md` renders correctly on the `main` branch.
• Docs rebuild successfully on Read the Docs with the new `/blob/main/` URLs.