wiki: add 126-plato, decommission 125-seafile

- containers/126-plato.md: Plato (notes/discovery) on LXC 126 — stack,
  bind mount, caddy + DNS, fresh-DB bootstrap workaround
- containers/125-seafile.md: removed (LXC destroyed 2026-05-13)
- containers/index.md: 126 added; 125 moved to recently-destroyed
- infrastructure/dns.md: plato entry added, files entry removed,
  Changelog reflects both
- README.md: top-level quick-links table swaps 125 → 126

README.md

@@ -31,7 +31,7 @@ See the full table in [`containers/index.md`](containers/index.md). Quick links:
 | 122 | [arriman](containers/122-arriman.md)               | 192.168.8.132 | Docker host (\*arr stack)                     |
 | 123 | [claudio-bot](containers/123-claudio-bot.md)       | 192.168.8.230 | Matrix control plane                          |
 | 124 | [authentik](containers/124-authentik.md)           | 192.168.8.180 | SSO + split-horizon DNS                       |
-| 125 | [seafile](containers/125-seafile.md)               | 192.168.8.185 | Seafile Pro (Nextcloud-replacement evaluation) |
+| 126 | [plato](containers/126-plato.md)                   | 192.168.8.190 | Plato (notes/discovery workspace)             |
 
 ### Cross-cutting infrastructure
 - [DNS — split-horizon](infrastructure/dns.md)