fix(ci): re-point validate-action references at the ecosystem repos (#198)

`hyperpolymath/k9-validate-action` and
`hyperpolymath/a2ml-validate-action` **no longer exist as standalone
repositories**. Both were consolidated into `k9-ecosystem` /
`a2ml-ecosystem` as real top-level directories and the originals
deleted.

**The content is intact** — 302 and 306 files respectively. Only these
references are stale.

Any workflow reaching them fails at job setup:

```
Unable to resolve action hyperpolymath/k9-validate-action, repository not found
```

## The fix — a path change, not a restore

GitHub resolves an action held in a **subdirectory** as
`owner/repo/path@ref`:

```diff
- uses: hyperpolymath/a2ml-validate-action@<old-sha>
+ uses: hyperpolymath/a2ml-ecosystem@aa4b836b
- uses: hyperpolymath/k9-validate-action@<old-sha>
+ uses: hyperpolymath/k9-ecosystem@89f3c270
```

Both targets verified to hold a real `action.yml` — *Validate A2ML
Manifests* and *Validate K9 Configurations*.

## How this stayed hidden

`dogfood-gate.yml` was itself **invalid YAML** in 71 repos, so the
workflow had never run and never attempted to resolve these actions.
Repairing the parse error exposed the stale reference underneath — one
layer of silent breakage concealing another.

## Verified before push

- every edited file still parses and still yields a `jobs:` mapping
- **no `*-validate-action@` reference remains**
- 1 file(s) changed, all under `.github/workflows`

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

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

Author: hyperpolymath

.github/workflows/dogfood-gate.yml

@@ -43,7 +43,7 @@ jobs:
 
       - name: Validate A2ML manifests
         if: steps.detect.outputs.count > 0
-        uses: hyperpolymath/a2ml-validate-action@0f8081cdfa293663ec6b204274b4272da302e564
+        uses: hyperpolymath/a2ml-ecosystem/validate-action@aa4b836bd969df2bc58128cb8e3d20bbc88d5e79
         with:
           path: '.'
           strict: 'false'