feat(config): add plugin_config for structured plugin options

[GoodFarming]

What does this PR do?

Adds an optional top-level plugin_config map to the config schema so plugins can receive structured options via opencode.json(c):

{
  "plugin": ["@acme/my-plugin"],
  "plugin_config": {
    "@acme/my-plugin": { "endpoint": "https://example.com" }
  }
}

Why?

Config parsing is strict (Config.Info is .strict()), so plugins can’t add plugin-specific keys to opencode.json(c) today without an upstream schema change. The common workarounds are env vars or sidecar config files, which are harder to install/standardize.

plugin_config is a minimal, additive escape hatch: OpenCode already passes the resolved config to plugins via Hooks.config, so plugins can read/validate their own entry without any new lifecycle plumbing.

Why this shape (vs plugin: [{ name, config }])?

Keeping plugin as a plain “load list” avoids changing plugin loading/deduping/merge semantics. plugin_config adds configuration without altering how plugins are resolved or initialized.

Notes

• plugin_config is deep-merged across config layers (same merge behavior as other config objects).
• Not intended for secrets; plugins should prefer existing auth/token flows when possible.

How did you verify your code works?

• bun test --cwd packages/opencode
• bun turbo typecheck
• bun ./script/generate.ts

Fixes #4393