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>

CONTRIBUTING.md

@@ -67,6 +67,30 @@ Links to nodes that host or depend on this.
 ## Changelog
 ```
 
+### Plan (`plans/YYYY-MM-DD-slug.md`)
+
+```markdown
+# YYYY-MM-DD — <title>
+
+## Goal
+What this change achieves and why.
+
+## Current topology / state
+Diagram or description of what exists now.
+
+## Target topology / state
+What it looks like after.
+
+## Pre-flight checklist
+
+## Step-by-step procedure
+
+## Verification
+
+## Post-migration
+Changelog entries to write, index status to update.
+```
+
 ### Investigation (`investigations/YYYY-MM-DD-slug.md`)
 
 ```markdown
@@ -89,6 +113,7 @@ Links to nodes that host or depend on this.
 - Every container page links to every cross-cutting page it participates in.
 - Every cross-cutting page lists the nodes that participate.
 - Every investigation links to the nodes it implicates *and* gets back-linked from each node's changelog.
+- Every plan links to the infrastructure pages it affects. When done, update the plan's status in `plans/index.md` and write changelog entries on affected node pages.
 
 ## Changelog hygiene