Marathon: copy trimmed session JSON

Marathon’s checkpoint prompt now supports /copy-trimmed, and the session actions menu (s → t) offers the same behavior. You still get the full session shape (state file or live snapshots, transcript, tools, and events), but long strings, fenced code blocks, and bulky tool parameters and results are truncated or replaced with compact placeholders.

This makes it more practical to share a run with another model or teammate for debugging and tuning, without manually editing megabytes of pasted JSON.