Add hooks for automatic code formatting on code change

[kevinher7]

Problem

Currently, opencode does not automatically format code when changes are made. This requires developers to manually run formatters after making edits, which can lead to inconsistent code style or forgotten formatting.

Proposed Solution

Add hooks that automatically format code whenever a file is changed/saved within opencode. This would:

• Integrate with existing formatters (prettier, black, clang-format, etc.)
• Run automatically on file save or after code generation
• Support configurable formatting rules per project
• Be opt-in or configurable to avoid interfering with existing workflows

Implementation Ideas

1. File watcher hook - Listen for file save/change events and trigger the appropriate formatter
2. Post-generation hook - Run formatter immediately after opencode generates or modifies code
3. Configuration file - Allow users to specify formatters per language/file type

Benefits

• Consistent code style across all changes
• Reduced manual steps for developers
• Cleaner commit history without separate formatting commits
• Better developer experience

Related

This would complement existing linting and type-checking capabilities.

[kevinher7]

Vibed too hard and created the issue here sorry

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The issue title should start with [FEATURE]: for feature requests (e.g., [FEATURE]: Add hooks for automatic code formatting on code change)
• Please use the Feature Request issue template, which requires a verification checkbox confirming the feature hasn't been suggested before and a structured description

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• On file edit it is also formatted #8387: opencode already formats files on edit — your request may overlap with existing formatter behavior or be a configuration request on top of it
• [FEATURE]: Add expanded hook events to the plugin system to enable more powerful plugin capabilities. #21075: Feature request for expanded hook events in the plugin system, which partially covers post-tool-use hooks that could enable custom formatting
• [FEATURE]: add OPENCODE_DISABLE_FORMATTER_DOWNLOAD env #22651: Feature request for formatter configuration/disabling, related to formatter integration

If you believe this was flagged incorrectly, please let a maintainer know.