Show prior conversation when resuming an agent-shell session

Set agent-shell-session-restore-verbosity to 'full so resuming a
session replays the previous prompts and responses into the buffer.

The default 'minimal restores context via session/resume, which the
agent uses to reload the conversation on its side but does NOT replay
any messages into the buffer -- so the model remembers everything yet
the buffer shows nothing, which is disorienting. claude-agent-acp
advertises session/load with full history replay, and 'full forces
that path, so the whole prior conversation is visible again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>