feat: add budget baseline command

[CoderDeltaLAN]

Summary

• Add read-only budget baseline command for deterministic local instruction-file metrics.
• Document output and exit-code contract updates.
• Add fixture-backed CLI and golden output coverage.

Validation

• python -m unittest tests.test_cli tests.test_golden_outputs
• ./scripts/check.sh
• python -m agent_rules_kit.cli budget tests/fixtures/repositories/multi-agent-overlap

Boundaries

• No tokenizer-specific token claims.
• No LLM, network, pricing, or optimization behavior.