fix(opencode): ACP Attach structured diff content for edit permissions

[MaxDillon]

Issue for this PR

Closes #11653

Type of change

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

What does this PR do?

This addresses an issue where ACP edit permissions do not include structured diff content, preventing ACP compliant editors to show diff before asking for final edit approval.

Previously, edit permissions only sent metadata (filepath, diff) to requestPermission, and the file was written immediately after approval. No ToolCallContent of type "diff" was attached, so the client could not render a diff preview.

This PR:

• Precomputes oldText and newText for edit permissions
• Attaches a { type: "diff", path, oldText, newText } object to requestPermission
• Writes the file only after approval using the same newText

This allows ACP clients (e.g. CodeCompanion) to render a proper diff preview.

Non-edit permissions are unchanged.

How did you verify your code works?

• I tested locally with opencode acp integration with CodeCompanion.nvim
• In the image section is the relevant configuration for hooking up local instance to codecompanion

Screenshots / recordings

Dev environment configuration

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Make change request

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Diff visualization in CodeCompanion.nvim

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Updated file after change approval

Checklist

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

[github-actions]

Hey! Your PR title ACP: Attach structured diff content for edit permissions 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.