feat(pi): add transparent resumable web sessions

[Dredok]

Summary

• add a pi_agent_owns_policy preference so Pi can keep ownership of its agent policy while Jean still supplies model, thinking, RPC, and session transport
• make Pi a first-class native terminal backend with stable session IDs, resume support, and importable history from ~/.pi/agent/sessions/**
• add preference, command-building, history, and frontend session tests plus architecture documentation

Why

Jean's web UI is useful as a persistent remote cockpit, but Pi should remain the actual agent harness. In transparent mode Jean provides the web/session transport without injecting its tool list or appended system policy into Pi.

Validation

• bun run test:run src/lib/native-cli-session.test.ts (8 passed)
• cargo check in jean-core (passed)
• stable v0.1.72 integration branch: full frontend typecheck/lint/Vitest, release server build, and clippy passed

Upstream baseline note

bun run typecheck currently stops in src/components/onboarding/OnboardingDialog.tsx:3368 with TS1005, a file unchanged by this PR and already present on main.