docs: document contributor branch policy

[abhiksark]

Summary

• make dev the integration target for feature and fix pull requests
• document squash merges into dev and merge-commit promotions to main
• link the contributor guide and release checklist as the sources of truth

Why

The contributor workflow and repository policy need to describe the release branch topology consistently before the v0.4.2 integration work begins.

Impact

Contributors now have one clear branch, pull request, and release flow. This is documentation-only and does not change runtime behavior.

Validation

• python -m pytest -q -> 151 passed
• pythonlings --root tests/fixtures/passing_curriculum verify -> passing1, passing2
• git diff --check -> passed

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