[BUG]: Partial formatter override silently dropped when command is not specified

[rcdailey]

Verify there isn't already a bug report for this.

• I have verified there isn't already an open bug report for this.

Describe the bug

When overriding a built-in formatter's extensions without also specifying command, the entire override is silently discarded and the built-in defaults (including all original extensions) remain active.

For example, this config intends to prevent prettier from formatting .md files:

"formatter": {
  "prettier": {
    "extensions": [".js", ".jsx", ".ts", ".tsx", ".css", ".scss", ".json", ".yaml", ".yml"]
  }
}

But prettier still formats .md files because the override is never applied.

Root cause

In packages/opencode/src/format/index.ts, user config is merged via:

const result: Formatter.Info = mergeDeep(formatters[name] ?? {}, {
  command: [],
  extensions: [],
  ...item,
})

if (result.command.length === 0) continue

The command: [] stub is spread before ...item. Since the user only provided extensions, command stays as []. mergeDeep uses replace semantics for arrays (not deep merge), so the empty array replaces the built-in command. The command.length === 0 guard then fires, skipping the override entirely. The original built-in formatter config (with all default extensions) remains.

Expected behavior

Partial overrides of built-in formatters should preserve unspecified fields. Overriding only extensions should keep the built-in command intact.

Workaround

Explicitly include the command in the override:

"formatter": {
  "prettier": {
    "command": ["npx", "prettier", "--write", "$FILE"],
    "extensions": [".js", ".jsx", ".ts", ".tsx", ".css", ".scss", ".json", ".yaml", ".yml"]
  }
}

OpenCode version

Latest (verified against current dev branch source)