docs: remove references to deleted scout agent

[JustSidus]

Issue for this PR

Closes #32105

Type of change

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

What does this PR do?

The scout agent was deleted in PR #30435 but the documentation still references it in multiple places:

• agents.mdx mentions Scout as one of three built-in subagents
• cli.mdx includes OPENCODE_EXPERIMENTAL_SCOUT in the environment variables table

This PR removes those outdated references to keep the docs in sync with the actual codebase.

Changes:

• Removed scout agent section from agents.mdx
• Updated subagent count from three to two
• Removed OPENCODE_EXPERIMENTAL_SCOUT from cli.mdx environment variables table

Note: Translations in other languages still reference scout agent but can be updated separately by translators.

How did you verify your code works?

• Reviewed the changes to ensure all scout agent references are removed from English documentation
• Verified the markdown syntax is correct
• Checked that the subagent count is consistent throughout the document

Screenshots / recordings

Not applicable - documentation change only.

Checklist

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