Monorepo: nested package's lint rules/jsPlugins silently dropped when workspace root has a lint block

## Summary

In a pnpm workspace/monorepo, when the **root** `vite.config.ts` has its own `lint` block, a **nested package's** `lint.rules` and `lint.jsPlugins` are silently ignored by `vp lint`. Remove the root's `lint` block and the nested config is forwarded correctly, so the nested config itself is valid.

Tested on vite-plus 0.3.0 (oxlint 1.79).

## Minimal reproduction

Self-contained, 6 files, no custom plugin needed. Run with `pnpm install` then `cd apps/child && vp lint`.

```
repro/
├── package.json # devDeps: vite-plus 0.3.0
├── pnpm-workspace.yaml # packages: ["apps/*"]
├── vite.config.ts # ROOT: has a lint block
└── apps/
└── child/
├── package.json # devDeps: vite-plus 0.3.0
├── vite.config.ts # CHILD: lint.rules + lint.jsPlugins
└── src/bad.ts # throws a string literal
```

Root `vite.config.ts`:

```ts
import { defineConfig } from 'vite-plus';

export default defineConfig({
lint: {
jsPlugins: [{ name: 'vite-plus', specifier: 'vite-plus/oxlint-plugin' }],
rules: { 'vite-plus/prefer-vite-plus-imports': 'error' },
options: { typeAware: true, typeCheck: true },
},
});
```

Child `apps/child/vite.config.ts`:

```ts
import { defineConfig } from 'vite-plus';

export default defineConfig({
lint: {
rules: { 'no-throw-literal': 'error' },
jsPlugins: [
{ name: 'ghost', specifier: 'this-plugin-is-definitely-not-installed' },
],
},
});
```

Child `apps/child/src/bad.ts`:

```ts
export function ping(): string {
throw 'a string literal'; // violates no-throw-literal ('ghost' jsPlugin would crash oxlint if loaded)
}
```

## Actual

```bash
$ cd apps/child && vp lint
# (no output — neither the rule nor the plugin is applied, and loading the
# nonexistent "ghost" plugin does not even get attempted)
```

## Expected

`throw 'a string literal'` should be reported by `no-throw-literal`, and the `ghost` jsPlugin load should be attempted (would expect a loud error for a nonexistent plugin).

## Proof the nested config is fine (and the bug is the root's lint block)

Root `lint` block present:

```bash
$ cd apps/child && vp lint
( no output )
```

Root `lint` block removed (only the `lint` block lines deleted):

```bash
$ cd apps/child && vp lint
src/bad.ts:2:9: error eslint(no-throw-literal): Expected an error object to be thrown
... [if child also has the ghost jsPlugins entry, it fails loudly instead:]
Failed to parse oxlint configuration file.
x Failed to load JS plugin: this-plugin-is-definitely-not-installed
```

Same behavior in `vp lint --print-config`: nested/root custom rules and jsPlugins never appear in the printed oxlint config when the root has a `lint` block.

Also worth noting: the same config in a **single-package (non-monorepo) project forwards `lint.jsPlugins` correctly**, so forwarding works in the simple case and breaks specifically with a root `lint` block present.

## Related

- #997 — nested package `ignorePatterns` dropped during config merging (closed; this may share the config-merge code path).

Start with the six-file reproduction, run pnpm install, then run cd apps/child && vp lint and vp lint --print-config with the root lint block present. Compare the printed configuration with the root and child vite.config.ts files; done means the child no-throw-literal rule and jsPlugins are forwarded or reported rather than silently dropped.

Written by the indexing model from the issue text.