Issue 16 OpenCode Ruff Hooks

Initial Situation

GitHub issue #16 reports reusable scaffold friction from dp scaffold setup in a backend plus frontend repository. OpenCode receives the auto-loaded .opencode/plugins/format-edited-file.mjs; it must not also receive a native formatter entry for the same runner. The shared neutral hook also needs .py edits to receive the same post-edit formatting/linting loop as JS/TS files.

Debt

Scaffold output implied hook coverage across generated agent surfaces, but the OpenCode and Python paths were incomplete:

• OpenCode plugin activation and native formatter configuration competed for ownership and could execute one edit twice.
• .agents/hooks/format-edited-file.mjs accepted JS/TS/JSON only.
• Python backend files needed Ruff routing through the owning Python project, not Oxlint/Oxfmt.

Bounded Fix

• Activate OpenCode only through its auto-loaded plugin mirror.
• Preserve existing OpenCode config while removing only the managed legacy harness-format-edited-file native formatter entry.
• Let the neutral hook accept .py files.
• Route captured Python bytes through uv run --project <python-project> ruff format --stdin-filename <file> -.
• Route the successful formatter bytes through uv run --project <python-project> ruff check --fix --stdin-filename <file> -; publish Ruff stdout only on success and otherwise preserve the formatter bytes while reporting lint failure.
• Scaffold the neutral hook when Python is detected, even if Oxfmt is absent.

Non-Goals

• Installing Ruff into consumer repositories.
• Replacing existing repo lint scripts.
• General Python lint asset authoring.
• Changing OpenCode model/provider settings.

Acceptance Checks

• Unit tests prove Python backend files resolve Ruff format/check commands.
• Projection tests prove OpenCode uses only the auto-loaded plugin and removes the managed native formatter entry.
• Scaffold setup tests prove Python-only repositories still receive the neutral hook.
• Docs/runbook mention OpenCode activation and Ruff routing behavior.