fix(pi): persist caller-owned sessions

[drewstone]

What changed

• Keep anonymous Pi requests ephemeral.
• Let a new caller-owned session create a persistent Pi session.
• Resume later turns through the stored Pi session ID.

Proof

• pnpm test: 435 passed, 6 skipped.
• pnpm run typecheck: passed.
• Real Pi/GLM-5.2 bridge smoke: two HTTP turns resumed one Pi session; SQLite recorded one mapping with turns=2.
• npx --yes @tangle-network/traces@latest analyze --harness pi --last 1 selected that exact session and captured two model events, 4,890 input tokens, 69 output tokens, and 0 execution failures.

This closes the trace blind spot exposed by the live agent-to-agent Runtime run.