chore(rulesets): retire deadlocked Optimus-Branch and adopt canonical base.json

[arena-ai-coding-agent]

Summary

This resolves the template artifact defect connected to #198 so freshly minted repositories from rsr-template-repo inherit valid, satisfiable rulesets rather than deadlocked configurations:

1. Removes .github/rulesets/Optimus-Branch.json: This file had previously been retired in chore(rulesets): retire Optimus-Branch from the template #97 (per upstream hyperpolymath/standards#789 ruling R1), but was accidentally resurrected during the merge of docs(machine-readable): point at the DEED grammar as the single normative source #76. It contained allowed_merge_methods: [], bypass_actors: [], require_code_owner_review: true, and required_approving_review_count: 2, which renders any newly created repo permanently unmergeable.
2. Adopts canonical .github/rulesets/base.json: Sourced directly from upstream hyperpolymath/standards/config/rulesets/base.json (§7.3). It configures the default branch baseline with squash merge, deletion protection, non-fast-forward protection, and valid bypass actors, without the retired/unsatisfiable code_coverage or code_quality rules.
3. Updates .github/rulesets/README.adoc: Corrects the file list and example POST/PUT commands to point to base.json and Immutable-Tags.json instead of the deleted tag-protection.json.

Verification

• bash scripts/validate-template.sh PASS (0 errors)
• bash tests/shape/check_root_shape_test.sh PASS
• bash tests/shape/repo_map_determinism_test.sh PASS

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