fix(core, opencode): add language consistency instructions to synthetic messages

[Grigory-Rylov]

Add language consistency instructions to compaction checkpoint, continuation prompt, and synthetic user messages so the model maintains the user's language after context compaction.

[github-actions]

Hey! Your PR title fix(core, opencode): add language consistency instructions to synthetic messages 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]

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