[FEATURE] Named checkpoints for rewind (label a point in conversation and jump back to it by name)

Feature request

Currently, the rewind menu (Esc Esc / \/rewind\) lists prior checkpoints by their message content and timestamp. There's no way to attach a custom label/name to a specific point in the conversation and later jump back to it by that name.

Use case

A common workflow: reach a point in the conversation (point A), do some open-ended interactive exploration, reach a conclusion (point B), and then want to either:

• rewind back to A and continue down a different path, or
• rewind back to A after distilling B's findings into a short summary, discarding the exploration but keeping the conclusion.

Right now this requires scanning the rewind list by eye to find the right point, since entries are only identified by raw message text/timestamp. As conversations get long, or when multiple candidate rewind points look similar, this becomes error-prone.

Proposed behavior

• Allow tagging/naming a checkpoint at the current point in conversation, e.g. via a command like \/checkpoint <name>\ or a keybinding.
• Named checkpoints show up in the rewind menu (Esc Esc) with their custom label instead of (or alongside) the raw message text, making them easy to find and jump back to.
• Nice-to-have: support this in headless/scripted usage too (relates to #16976), so a checkpoint could be referenced by name rather than only by UUID.

Why this matters

This complements existing rewind/checkpoint functionality (#16976, #43755) by making checkpoints addressable and legible over long sessions, rather than relying on visually scanning raw conversation snippets to find the right point to rewind to.