[FEATURE]: Add ability to (un)mark points in the thread, to use as "bookmarks"

[alexaandru]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The idea is plainly stolen from vim: add ability to set/unset marks in the thread. Related to it, would be the (optional?) display of marks in the thread (a "hr" ------- or something).

The usecase is: keeping track with what was reviewed so far in the thread, mark it, then do something else, then when you come back to opencode, either scroll back up to that mark or, even better, be able to jump straight to the mark (optionally remove it), review the new stuff, repeat.

The smarter the LLMs become -> the more they can work on their own -> the more important is to have some good tools to manage navigation of the consequently longer threads. I think marks+jumps would be an excellent addition.

Optionally, ability to annotate the marks with a message would also be useful. Optionally 2, having the marks indicated in the scrollbar itself, would also be nice :-)