Marathon: session trees, mid-turn steering, and the context ledger

Marathon's long-running agent loop just got a durable memory and a steering wheel. Every session now appends to a tree-structured log, so you can branch, rewind, and redirect without losing anything.

• Session trees. Use /tree to browse a session's full history and jump the conversation back to any earlier point. /fork branches from a prior message so you can try a different direction; the original timeline always survives.
• Mid-turn steering. Press Enter while the agent is working to queue guidance. It lands within seconds or after the current work finishes, your choice. Double-Esc aborts to a resumable checkpoint.
• Context ledger. Trimming the context window no longer means losing data. Tool outputs and compaction summaries are stored durably; only what the model sees each turn is reduced, and true sizes drive compaction decisions.
• Recall by default. Session search now indexes offloaded tool outputs and compaction summaries, so anything trimmed for context stays findable.

Long marathons stay steerable, inspectable, and reversible.