Build step (npm run build) mutates template source files and causes data loss

### Describe the bug
Running `npm run build` in the AsyncAPI Generator repository **silently rewrites source-controlled template files** under `packages/templates/*`.

The build script (`scripts/build-templates.js`) overwrites:
- `.ageneratorrc` (replaces the entire `metadata` object)
- `package.json` (forces the template `name`)

This happens **without warning, confirmation, or opt-in flag**. Any manual changes made by contributors to template metadata or package identity are **lost permanently** after running the build. Since the build also runs as part of tests and CI, this behavior is highly surprising and dangerous.

A build step is expected to be **read-only with respect to source files**, but currently it mutates them.

---

### Steps to reproduce

1. Go to the generator repository root:
```bash
cd generators/generator
2. Pick an existing baked-in template, for example:
```
packages/templates/clients/websocket/javascript
```

3. Edit .ageneratorrc and add a custom field:
```
metadata:
type: client
protocol: websocket
target: javascript
customField: SHOULD_SURVIVE_BUILD
```
4. Edit package.json and change the name:
```
{
"name": "my-custom-ws-js-template"
}
```
5. Verify changes exist:
```
git diff
```
6. Run the build:
```
npm run build
```
7. Check the diff again:
```
git diff
```
## Current behavior
- `.ageneratorrc` is rewritten and `customField` is removed
- package.json.name is reset to the auto-generated value
- No warning or prompt is shown
- Git diff is clean, hiding that changes were overwritten

## Expected behavior
- npm run build should not modify template source files
- Template normalization (if needed) should be:
- opt-in (e.g. --fix, --write)
- or handled by a separate maintenance script
- Existing metadata should be validated or merged, not overwritten
- package.json.name should never be rewritten silently

### I am willing to submit a PR that

Read scripts/build-templates.js first, then reproduce the issue from the repository root with npm run build and inspect git diff. Focus on packages/templates/*, especially .ageneratorrc and package.json; done means the build leaves source files unchanged and preserves custom metadata and the template package name.

Written by the indexing model from the issue text.