fix(ci): remove placeholder entry blocking Dependabot (#24)

This `dependabot.yml` carries GitHub's **unedited placeholder**
alongside a real, correct entry:

```yaml
  - package-ecosystem: ""   # See documentation for possible values
```

An empty `package-ecosystem` fails Dependabot's schema validation, and
GitHub rejects the **whole file** — not just the bad entry. So the real
`github-actions` entry has never run either.

### Measured, not assumed

The three estate repos in exactly this state have **0 Dependabot PRs
between them**, while comparable repos with valid configs have plenty:

| repo | config | Dependabot PRs |
|---|---|---|
| `blocky-writer` | stub + real entry | **0** |
| `jtv-lang` | stub + real entry | **0** |
| `lcb-website` | stub + real entry | **0** |
| `aerie` | valid | 7 |
| `standards` | valid | 8 |
| `affinescript` | valid | 16 |
| `hypatia` | valid | 63 |

All three do have a populated `.github/workflows/`, so there is
genuinely something for `github-actions` to update.

### Fix

Excise the stub block. The real entry is untouched and becomes live.
Where an entry lacked a `groups:` block it gains one, so an
ecosystem-wide bump arrives as **one** PR instead of one per dependency.

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

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/dependabot.yml

@@ -5,10 +5,6 @@
 
 version: 2
 updates:
-  - package-ecosystem: "" # See documentation for possible values
-    directory: "/" # Location of package manifests
-    schedule:
-      interval: "daily"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: