site: add remote backends announcement post (#187)

## Summary
- New announcement post at
`site/public/announcements/remote-backends.html` covering the remote
backends feature: one-click SSH bootstrap (Add Backend → SSH host tab),
the manual headless-server setup, feature grid, and security notes
- Animated hero diagram (Harness app + phone web client → ssh · http →
harness-server → agent worktree rows) built in pure CSS, with
`prefers-reduced-motion` support
- Two images: the rubber-hand laptop photo (intro joke) and the Add
backend SSH-host modal screenshot, both converted to webp
- Prepends the post to `site/public/announcements.json` (newest first)

## Test plan
- [x] Served `site/public` locally and verified layout, animation, and
images in a browser
- [x] Console clean apart from the expected Tailwind CDN warning
- [x] `announcements.json` parses (`node -e "JSON.parse(...)"`)
- [ ] Verify the announcements list page renders the new entry after
deploy

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

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: frenchie4111

site/public/announcements.json

@@ -1,5 +1,12 @@
 {
   "announcements": [
+    {
+      "id": "remote-backends-2026-07",
+      "title": "Run your agents on any machine",
+      "href": "https://harness.mikelyons.org/announcements/remote-backends.html",
+      "publishedAt": "2026-07-16",
+      "summary": "Connect the Harness desktop app to a headless server on another machine. One-click SSH setup, switch backends with a keystroke, notifications from every machine."
+    },
     {
       "id": "introducing-chat-mode-2026-05",
       "title": "Introducing Chat mode",