Add agent-shell-restore-context defcustom

[nhojb]

Currently when restoring a session you can choose a minimal resume or a full load via agent-shell-prefer-session-resume.

This PR adds a summary option via a new agent-shell-restore-context defcustom. The options are:

• minimal: Equivalent to agent-shell-prefer-session-resume: t
• full: Equivalent to agent-shell-prefer-session-resume: nil
• summary: Loads the full session (when available), but renders only the first prompt and last text response.

The idea is that summary gives you enough context about the session, without needing to restore the entire conversation and tool calls.

Here's an example:

Checklist

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