Migrate default branch from `master` to `main`

## 📌 Description

The repository currently uses the `master` branch as its default. To align with modern, inclusive standards and GitHub’s default conventions, we need to:

* Officially deprecate `master`
* Transition to `main` as the new default branch
* Update all references across the repo and community workflows

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## 🎯 Rationale

* **Inclusivity**: Industry-wide shift to using `main` instead of `master`.
* **Consistency**: Aligns Beckn Protocol repos with open-source best practices.
* **Compatibility**: Many CI/CD and automation tools expect `main` as the default.

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## ✅ Tasks

* [ ] Change default branch in GitHub repository settings from `master` → `main`.
* [ ] Ensure `main` is protected with the same branch protection rules as `master`.
* [ ] Update references to `master` in:

* [ ] Documentation files
* [ ] CI/CD configurations
* [ ] Contribution guidelines
* [ ] Governance references
* [ ] Notify community via Announcement (see linked PR/Discussion).
* [ ] Add redirects/aliases where necessary to avoid broken links.

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## 🔄 Community Instructions

Contributors are requested to:

* Update local branches (`git branch -m master main`).
* Rebase any open PRs onto `main`.
* Update Issues, Discussions, and documentation references pointing to `master`.

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## 📎 References

* [GitHub guidance on renaming branches](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch)

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🔔 **Impact**: Medium — Requires community coordination but no breaking changes to specifications.