Fix ReferenceError where __dirname is not defined in ES modules by
importing fileURLToPath and calculating __dirname from import.meta.url.

Also update generated mise.ts file from render task.

Fixes autofix CI failure in #7010

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

## Summary

- Fix ReferenceError where `__dirname` is not defined in ES modules
- Add ES module-compatible `__dirname` calculation using
`import.meta.url`
- Update generated `mise.ts` file from render task

## Problem

The autofix CI job was failing in PR #7010 with:
```
ReferenceError: __dirname is not defined
    at main (/home/runner/work/mise/mise/xtasks/fig/addCustomGenerators.ts:150:40)
```

This occurred because `addCustomGenerators.ts` is an ES module (imported
with `import`), but was using `__dirname` which is only available in
CommonJS modules.

## Solution

Import `fileURLToPath` from `node:url` and calculate `__dirname` from
`import.meta.url`:

```typescript
import { fileURLToPath } from "node:url";

const __dirname = path.dirname(fileURLToPath(import.meta.url));
```

This provides ES module-compatible access to the current directory path.

## Test Plan

- [x] Run `mise run render` to verify the fig generation works
- [x] Run `mise run lint-fix` to ensure all linting passes
- [x] Verify pre-commit hooks pass

## Changes

- `xtasks/fig/addCustomGenerators.ts`: Add ES module-compatible
`__dirname`
- `xtasks/fig/src/mise.ts`: Regenerated file from render task

Fixes autofix CI failure in #7010

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude <noreply@anthropic.com>