v1.51.1 — Pi Extension Fix

What's Fixed

• #284: Pi extension is now compiled and included in the published npm package
  • Extension rewritten against pi's real ExtensionAPI (was using a fictional API)
  • Build script compiles packages/pi-extension as part of the main build
  • pi.extensions path updated to point to compiled `dist/index.js"
  • packages/pi-extension/dist included in npm tarball files array
  • All 8 slope tools available: slope_briefing, slope_card, slope_guard_check, slope_sprint_context, slope_sprint_validate, slope_review_run, slope_guard_metrics, `slope_convergence"
  • Guard events: commit discipline, post-push nudge, hazard warnings
  • Auto-injected sprint briefing on first turn via `before_agent_start"