Add release changelog agentic workflow (#638)

* 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>

* Address PR review feedback

- Add explicit guidance for workflow_dispatch runs where release event
  fields are empty (fetch release by tag via API)
- Use release publish date instead of today's date for changelog entries
- Remove author attribution to match example style (keep it clean)

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

---------

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

Author: SteveSandersonMS

.github/aw/actions-lock.json

@@ -0,0 +1,29 @@
+{
+  "entries": {
+    "actions/checkout@v6.0.2": {
+      "repo": "actions/checkout",
+      "version": "v6.0.2",
+      "sha": "de0fac2e4500dabe0009e67214ff5f5447ce83dd"
+    },
+    "actions/download-artifact@v8.0.0": {
+      "repo": "actions/download-artifact",
+      "version": "v8.0.0",
+      "sha": "70fc10c6e5e1ce46ad2ea6f2b72d43f7d47b13c3"
+    },
+    "actions/github-script@v8": {
+      "repo": "actions/github-script",
+      "version": "v8",
+      "sha": "ed597411d8f924073f98dfc5c65a23a2325f34cd"
+    },
+    "actions/upload-artifact@v7.0.0": {
+      "repo": "actions/upload-artifact",
+      "version": "v7.0.0",
+      "sha": "bbbca2ddaa5d8feaa63e36b76fdaad77386f024f"
+    },
+    "github/gh-aw/actions/setup@v0.52.1": {
+      "repo": "github/gh-aw/actions/setup",
+      "version": "v0.52.1",
+      "sha": "a86e657586e4ac5f549a790628971ec02f6a4a8f"
+    }
+  }
+}