v2: generic tool output truncation drops the tail (skills, MCP results)

Description

When a tool result exceeds tool_output (default 2,000 lines / 50 KiB), ToolOutput.truncate keeps only the first lines. Everything after the cut never reaches the model.

Shell output already keeps the end: packages/core/src/shell.ts pages from size - maxBytes and reports the last line range. Other tools keep the start. That asymmetry is the problem:

• a large SKILL.md whose final checklist or "do not" rules sit at the bottom
• an MCP tool whose result field is at the end of a long pretty-printed response
• a long build log where the error is in the last few lines

The marker does tell the model that the full output was saved to a file, but in practice it often continues without reading it.

This is the same shape as #54148 (closed as not_planned because it was reported on v1; the bot asked for a v2 report and a link back). Related: #35661 (MCP output truncated at 50 KB). Reproduces on current dev.

Steps to reproduce

1. Configure tool_output: { "max_lines": 10 }.
2. Call a tool or plugin that returns 100 numbered lines (or load a SKILL.md larger than 50 KiB).
3. The model receives lines 1-10 plus the marker; lines 11-100 never reach it.

OpenCode version

0.0.0-dev (v2 dev @ da8c0211f7)

Operating System

Linux

Terminal

Konsole

Suggested direction

Keep both ends instead of only the head:

• head and tail share max_lines
• the marker keeps line ranges, e.g. [showing lines 1-5 and 96-100 of 100; ...], so it still matches read's offset
• attachments keep their original relative position
• the full output is still written to the retained file

If generic head+tail is unwanted, a narrower option is to keep the tail only for MCP/skill results, but the generic truncation path is where the loss happens.