resolveExecPrompt spawns x/prime which uses envsubst to expand
${OLLIE_SESSION_ID} in the system prompt template. The variable was
never in the process environment at resolution time — it was only
set on the execute server after the prompt was already rendered.
Add env []string parameter to resolvePrompt/resolveExecPrompt and
BuildAgentEnv so callers can inject session-scoped vars into the
subprocess environment. Generate session ID before BuildAgentEnv in
the agent-switch path so it is available at prompt resolution time.
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