Add release changelog agentic workflow

Create an AI agent that automatically generates CHANGELOG.md entries
when a stable (non-preview) release is published. The agent:

- Triggers on release:published events (filtered to non-prerelease)
- Reads merged PRs/commits between the new and previous stable tags
- Generates categorized, bullet-pointed changelog entries
- Opens a PR to update CHANGELOG.md via create-pull-request safe output
- Updates the GitHub Release notes via update-release safe output
- Posts an announcement Discussion for team visibility

Also adds a seed CHANGELOG.md at the repo root.

Note: The .lock.yml must be compiled via 'gh aw compile release-changelog'
before this workflow will run in CI.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

.github/aw/actions-lock.json

@@ -0,0 +1,14 @@
+{
+  "entries": {
+    "actions/github-script@v8": {
+      "repo": "actions/github-script",
+      "version": "v8",
+      "sha": "ed597411d8f924073f98dfc5c65a23a2325f34cd"
+    },
+    "github/gh-aw/actions/setup@v0.52.1": {
+      "repo": "github/gh-aw/actions/setup",
+      "version": "v0.52.1",
+      "sha": "a86e657586e4ac5f549a790628971ec02f6a4a8f"
+    }
+  }
+}