docs: Add experimental Plan mode option to CLI documentation

[raulpesilva]

Update docs for the new experimental Plan mode

What does this PR do?

• add OPENCODE_EXPERIMENTAL_PLAN_MODE feature flag documentation

[github-actions]

Hey! Your PR title doc: Add experimental Plan mode option to CLI documentation doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found