fix(opencode): allow partial command overrides in JSON config

[joao-paulo-santos]

Issue for this PR

Fixes #27035

Type of change

• Bug fix

What does this PR do?

ConfigCommand.Info requires template as a mandatory field, but the docs show that JSON config can override just the model for a built-in command. Since template is derived from markdown content for built-in commands, users shouldn't need to copy-paste it.

Made template optional in the Info schema. Markdown-loaded commands always set template from md.content.trim(), so they're unaffected. The command/index.ts loop now skips entries without a template — partial overrides get their template filled in when markdown commands are merged at config.ts:619.

Note: This PR is scoped to fixing the schema validation only. The actual override values (e.g. model) are currently overwritten by the markdown merge at config.ts:619 due to merge order (mergeDeep(jsonConfig, markdownCommands) — markdown wins on overlapping keys). That should be addressed as a separate issue.

How did you verify your code works?

• bun test test/config/config.test.ts — 84 tests pass, including a new test that configures a command override with only model (no template)
• bun turbo typecheck — 14/14 packages clean
• Pre-push CI hook confirmed typecheck passes

Screenshots / recordings

N/A — config validation fix, no UI changes.

Checklist

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

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