fix(harness): reload continuation state after Substrate restore (#2823)

After #2715 , Substrate can restore `/data` into a process resumed from
a golden snapshot after the continuation store is initialized. Codex and
Claude were reading stale in-memory state that was preserved in the
golden snapshot (with an empty initial session ID), despite `state.json`
containing the correct native session ID. As a result, each subsequent
turn started a new session and overwrote the restored continuation
state.

This change makes `state.json` authoritative by reloading it on every
`Load` and `Bind`.

Signed-off-by: Jet Chiang <pokyuen.jetchiang-ext@solo.io>