feat: add pi-native Signum runtime MVP

[11me]

Summary

• add first-class pi support directly in this repo via a native /signum command
• implement the pi runtime flow through CONTRACT -> EXECUTE -> AUDIT -> PACK
• package the repo root as a pi-installable npm package and add pi smoke coverage
• include verify-dialect normalization to stabilize pi contract verification inputs

Included

• root package.json with pi extension manifest
• platforms/pi/ native extension surface, role prompts, and runtime helpers
• CONTRACT, EXECUTE, AUDIT, and PACK phases in the pi runtime
• runtime policy-wrapped engineer tools and execute receipts
• pi explain/init/archive/close support
• pi smoke tests and packaging checks
• pi verify normalizer and coverage test

Validation

• npm run check
• live pi smoke for /signum <task> with dev auto-approve

Notes

• AUDIT is currently single-pass in the pi runtime
• iterative audit parity is intentionally deferred as follow-up work