fix: skip load for pristine sessions

[lynnzc]

Summary

• skip session/load for pristine saved sessions that have no turns yet and only need a fresh ACP session
• keep existing resume behavior for sessions with history, explicit mode state, or config replay needs
• add a regression test covering the pristine-session fast path

Why

Issue #165 reports a large delay before the first streamed text event on codex prompt --session compared with codex exec.

For freshly ensured sessions with no prior turns, acpx was still attempting session/load before the first prompt. On Codex, that extra resume path appears to be the wrong fit for a pristine session and can delay the first streamed output. This change sends those pristine sessions straight to session/new instead.

Fixes #165.

Validation

• pnpm run build
• pnpm run build:test
• node --test dist-test/test/connect-load.test.js
• node --test dist-test/test/prompt-runner.test.js