[repo] fix : remove branch policy workflow

[jischeng]

Summary

• remove the repository-local branch policy workflow and validation script that were introduced by #8
• keep the GitHub repository rulesets for master and */main-* branch protection
• update the branch policy documentation to reflect that source-to-target branch flow is now a convention, not a repository check

Why

The workflow added in #8 enforced the head-to-base branch flow inside CI. After it was merged, it immediately blocked follow-up pull requests that were needed to bootstrap and adjust the policy itself. We want to keep the GitHub-side rulesets, but stop enforcing the source-branch restriction from inside the repository.

Testing

• git diff --check -- .claude/learnings.md docs/development/github_branch_policy.md .github/scripts/validate_branch_policy.py .github/workflows/branch_policy.yml

Changelog category (leave one):

• Not for changelog

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

• N/A

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Follow-up to #8.