Pin agent responses in a thread and jump between pins

[caitlon]

Problem. Long threads bury the few answers that matter: the agreed plan, a root-cause explanation, the final command list. Finding them again means scrolling or searching for words I may not remember.

Proposal. A pin action on any agent (or user) message. Pinned messages appear in a small per-thread list (a header popover or a side-panel tab), each entry showing the first line and its time. Clicking an entry scrolls to the message; next/previous-pin keybindings move between them. Pins are stored with the thread, so they survive reloads and are visible from every client connected to the same environment.

Prior art. Claude Code in the Claude desktop app lets you pin messages in a conversation and jump back to them.

Related. Search within a thread (#6709) finds text but needs the right words; minimap prompt navigation (#6763) steps through every prompt; the sticky latest prompt (#6946) covers only the newest user message. Pins cover the case in between: a handful of messages the user chose.