Pitch

DeepSeek-unique. We have `cycle_manager.rs` with cycle-boundary tracking but it's invisible to the user. Surface it inline:

• A subtle horizontal divider between turns when a cycle boundary fires.
• Header chip on the new cycle: `──── cycle 2 ────` in TEXT_DIM.
• `/cycles` slash command lists boundaries with their summaries.
• Tooltip on the divider explains what cycles do (one-line: "context summary checkpoint").

This makes a long session legible — users see the natural breaks instead of a 4000-line wall of text.

Acceptance

• Cycle boundaries render as visible dividers in the transcript.
• Cycle 1 doesn't render a divider (no boundary above it).
• /cycles shows the list with timestamps.
• Doesn't disrupt scroll / selection.