multiple formatters (prettier + biome) run in sequence and fight over the file

Describe the bug

getFormatter (packages/opencode/src/format/formatter.ts) returns every enabled formatter whose extension matches, and formatFile runs them all in sequence. When both Prettier and Biome are enabled for the same extension (a very common project setup), the file is rewritten twice and the two formatters fight over style, producing flip-flop diffs on every edit. The final on-disk content depends on formatter registration order, so it can disagree with what the model just wrote (and edit re-reads the overwritten content).

The code is aware of this hazard — uvformat explicitly returns false when ruff is enabled — but there's no mutual exclusion between prettier/biome/oxfmt or rubocop/standardrb.

Suggested fix

Pick a single winning formatter per file (configurable precedence) instead of running all matches, or skip a formatter when a competing one already handled the file.

Environment

• opencode version: latest dev