[architect] validate-metrics.mjs has malformed shebang (missing /)

## Architecture Finding

**Type**: code-defect / script-portability
**Affected area**: `scripts/validate-metrics.mjs` — line 1

The shebang line reads:
```
#!/usr bin/env node
```
The `/` between `usr` and `bin` is missing. The correct form is `#!/usr/bin/env node`.

## Impact

- `./scripts/validate-metrics.mjs` run directly fails (the kernel cannot find the interpreter path `/usr bin/env`).
- `npm run validate:metrics` is unaffected — npm always invokes `node scripts/...` directly, so the shebang is not parsed.
- CI is working correctly. This is a script-portability defect that only surfaces when scripts are executed directly.

## Fix

```diff
-#!/usr bin/env node
+#!/usr/bin/env node
```

---
*Filed by architect agent (ACMM L5)*