Pi runtime front door and install UX

[clkao]

Pi users can now launch, check, and validate Spacedock workflows through a native Pi runtime path.

What changed

• Added Pi-native dispatch and runtime adapter contracts.
• Added live Pi subagent smoke coverage.
• Added spacedock pi, Pi install, and Pi doctor UX.
• Hardened Pi setup flag behavior and docs.
• Documented stage dispatch without subagent acceptance contracts.

Evidence

• Go baseline and race suites: 2/2 passed.
• Focused Pi CLI suites and live Pi smoke: passed.

---

mf
Related: pi-runtime-support
Closes #147