Implement BranchProtectionAssessor using GitHub API

[jwm4]

Background

BranchProtectionAssessor (src/agentready/assessors/testing.py) is registered in create_all_assessors() but its assess() method unconditionally returns not_applicable. It has never contributed to any assessment score.

The assessor's own docstring describes the intended implementation:

This is a stub implementation that will return not_applicable until GitHub API integration is implemented.

What it should check

Per the existing criteria in the assessor:

• Required status checks on the default branch
• Required review approvals before merging
• Force push prevention
• Branch update requirements (branch must be up to date before merging)

Implementation notes

• The project already has GitHub integration infrastructure in src/agentready/github/
• The assessor is Tier 4 (Advanced) with default_weight=0.005 — low weight, low risk
• Should gracefully degrade to skipped when no GitHub token is available or the repo is not on GitHub (e.g., local-only repos, GitLab)
• branch_protection is also absent from default-weights.yaml (tracked in Fix default-weights.yaml: dead key, missing entries, and scoring drift #403) — that should be resolved alongside or before this

Acceptance criteria

• assess() returns a real pass/fail finding when GitHub API access is available
• Returns skipped (not not_applicable) when no token or non-GitHub remote is detected
• Unit tests cover pass, fail, and skipped cases
• branch_protection added to default-weights.yaml (see Fix default-weights.yaml: dead key, missing entries, and scoring drift #403)

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Opened by Claude Code under the supervision of Bill Murdock.