feat: add mix lc development proxy

[bougyman]

Summary

• add a root mix lc task that runs the development CLI from app/
• forward argv and inherit stdin, stdout, and stderr for true interactive behavior
• suppress child Mix progress while preserving the exact CLI exit status
• document mix lc as the canonical local dogfooding command

Verification

• mix test (11 tests)
• mix format --check-formatted
• mix usage_rules.sync --check from app/
• mix lc version --output json returns clean JSON
• interactive project selection completes through mix lc
• a CLI validation failure propagates exit status 22

Linear: CRY-43

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