network: retire Slate AX; add SODOLA switch migration plan and network topology doc

- infrastructure/network.md: permanent topology reference — VLAN 1 (main LAN)
  + VLAN 10 (homelab 192.168.8.0/24), Fritz!Box 7590 as VLAN router,
  SODOLA 5-Port 2.5Gbit as homelab switch
- plans/2026-06-01-slate-ax-to-sodola-migration.md: full migration runbook
  (pre-flight, Fritz!Box second-network config, SODOLA VLAN table, cutover
  procedure, verification commands). Gateway 192.168.8.1 is unchanged —
  no LXC/VM config edits needed. Fritz!Box DHCP for VLAN 10 hands out
  192.168.8.2 (Technitium CT 107) as DNS, closing the outstanding item in dns.md.
- plans/index.md: establishes plans/ convention (Planned → In Progress → Done)
- CONTRIBUTING.md: plan page template + linking discipline for plans

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/index.md

@@ -0,0 +1,16 @@
+# Plans
+
+Pre-flight runbooks for planned changes that haven't happened yet. Once executed, move the outcome to `investigations/` (if anything interesting happened) or just a changelog entry on the affected node pages.
+
+## Index
+
+| Date | Title | Status |
+| ---- | ----- | ------ |
+| 2026-06-01 | [Slate AX → SODOLA managed switch migration](2026-06-01-slate-ax-to-sodola-migration.md) | Planned |
+
+## Conventions
+
+- File name: `YYYY-MM-DD-<slug>.md`. Use the *target* date if known, otherwise the planning date.
+- Status: `Planned` → `In Progress` → `Done` (update index entry as it moves).
+- When done: add a changelog entry on every affected node page, then update status to `Done` here. If things went sideways, open an investigation.
+- Plans are append-only once execution starts — don't rewrite pre-flight intent after the fact.