[Gleek]

In chat mode, streaming responses cause frequent relabeling of the entire conversation. This is expensive in long sessions: my CPU profiles showed prompt discovery taking roughly half the sampled CPU time. With multiple such sessions running in parallel Emacs bogs down quite considerably.

21118  79% - timer-event-handler
21089  79%  - apply
12639  47%   - agent-shell-chat--relabel-buffer
12635  47%    - agent-shell-chat--relabel
11962  44%     - agent-shell-chat--label-prompts
11584  43%      - agent-shell-chat--prompt-runs
 8416  31%         next-single-property-change
 2907  10%         agent-shell-chat--prompt-face-p

Disclaimer: agent-shell is v0.87.10

From what I understand the prompt overlays include text inserted at their start.

Since streamed output is inserted immediately before the live prompt, those overlays can expand over the output and hide it. Relabeling adjusts their boundaries to reveal the output.

This change gives prompt overlays front-advance so they move with the prompt automatically.
Ordinary streaming skips prompt relabelling, while response headings continue to update.
Submissions, restores, etc, would retain full relabelling.

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