Long region preview's "Expand..." button is sent literally to the agent

[alberti42]

When I send a region longer than 5 lines (e.g. via agent-shell-send-region), the inserted preview is capped and followed by an Expand... button (agent-shell.el:6113-6135, max-preview-lines 5). If I press RET to submit without first clicking the button, the agent receives the truncated preview plus the literal string Expand... — the full text never reaches it.

Cause: the full text is captured only inside the button's action closure. shell-maker-submit → comint-send-input reads the prompt with buffer-substring-no-properties, so the closure is never consulted.

Possible fixes:

1. Auto-expand any collapsed agent-shell-region-id regions in the input area before submission.
2. Store the full text on a text property (or invisible overlay) that submission can substitute back in, so the visible preview stays compact but the agent receives the full content.
3. At minimum, expose max-preview-lines as a defcustom so users can disable the cap.

Repro:

1. Open a buffer with > 5 lines of text, mark a region spanning all of them.
2. M-x agent-shell-send-region.
3. Press RET without clicking Expand....
4. Observe that the agent sees …\nExpand... instead of the full region.

Checklist

• I agree to communicate with the author myself (not AI-generated).
• I've read the README's Filing issues section.
• I'm running the latest versions (fill in below).
• Emacs: GNU Emacs 31.0.60
• agent-shell: 67babec (2026-05-16, merge of Add Hermes Agent Support  #583)
• acp.el: 03099fa (2026-05-03, merge of Add support for devcontainer #18)
• ACP package: OpenCode adapter (agent-shell-opencode.el, bundled with agent-shell)
• Agent CLI: opencode 0.0.0-integrated-202605150926 (personal fork with --attach support; see config below)
• For requesting new agent support, I'm including a link to the ACP-capable agent or related ACP package.
• For issues, I'm including ACP traffic (as per README).

-- Thanks a lot for your consideration! Andrea