docs(config): document the tool_output truncation thresholds

[harshil-mistry]

Issue for this PR

Closes #45232

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

tool_output has worked since #23770 (merged 2026-04-22) and appears nowhere in the docs. Searching packages/web/src/content/docs/ for tool_output, max_lines or max_bytes returns nothing.

That matters because the behaviour it controls is visible and often unwanted: once a tool's output crosses 2000 lines or 50 KB, the model gets a preview and a file path instead of the output. The only knob for that was undiscoverable — this adds a ### Tool output section to config.mdx with an example, both fields, their defaults, and the fact that whichever limit trips first wins.

Defaults quoted from the schema annotations in packages/core/src/v1/config/config.ts (2000 lines, 51200 bytes), which match the MAX_LINES / MAX_BYTES constants in packages/opencode/src/tool/truncate.ts.

Placed before ### Sharing so it sits next to the other runtime-behaviour options rather than in with the provider/model sections.

How did you verify your code works?

I diffed all 36 top-level keys in the v1 config schema against config.mdx to check this was a real gap and not something documented under another name:

NOT mentioned anywhere in config.mdx:
  autoshare, config, enterprise, layout, mode, reference,
  references, remote_config, skills, tool_output, username
25/36 documented

tool_output genuinely appears nowhere. I confirmed the values I wrote by tracing the read path — truncate.ts resolves cfg?.tool_output?.max_lines ?? MAX_LINES and cfg?.tool_output?.max_bytes ?? MAX_BYTES, with MAX_LINES = 2000 and MAX_BYTES = 50 * 1024, and output() truncates when lines.length > maxLines || totalBytes > maxBytes, which is what "whichever limit is reached first" describes.

I deliberately kept this to tool_output rather than closing all eleven gaps at once. Several of the others (autoshare, mode, layout) are legacy keys that probably should not be documented, and a bulk sync is a different kind of change. Happy to do the remaining live ones separately if you want them.

Screenshots / recordings

Docs change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR