feat(hooks): add merge-conflict guardrail

[azalio]

Summary

• add MAP conflict-resolution guidance to the workflow-context injector for git merge/rebase preflight and active unmerged indexes
• detect conflicted paths read-only via git diff --name-only --diff-filter=U -z -- and keep the existing step_state.json gate
• document the per-file, intent-preserving, test-after-each-batch protocol

Testing

• uv run pytest tests/test_workflow_context_injector.py -v
• make check-render
• make check

Closes #256

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