settings(template): default + fork-deep to Claude Opus 5

pi 0.82.0 added Claude Opus 5 (Anthropic + Bedrock, adaptive thinking incl.
xhigh, 1M ctx / 128K out). Point the template at it:

- defaultModel: eu.anthropic.claude-opus-4-8 -> eu.anthropic.claude-opus-5
- enabledModels: add opus-5 at the head, drop opus-4-7 (superseded; 4-8 kept
  as the previous-gen fallback so the picker stays at four entries)
- pi-fork deep tier: opus-4-8 -> opus-5 (fast=haiku-4-5, balanced=sonnet-5
  unchanged)

README's settings-template snippet updated in the same commit to stay in sync.

Fresh machines / fresh pi-devbox volumes get this verbatim via the entrypoint
bootstrap. Existing volumes are unaffected: entrypoint-user.sh deep-merges
template-FIRST, live-SECOND with arrays as leaves, so live values win and no
model a user removed is re-added.

README.md

@@ -176,10 +176,10 @@ pi --version
 ```json
 {
   "defaultProvider": "amazon-bedrock",
-  "defaultModel": "eu.anthropic.claude-opus-4-8",
+  "defaultModel": "eu.anthropic.claude-opus-5",
   "enabledModels": [
+    "eu.anthropic.claude-opus-5",
     "eu.anthropic.claude-sonnet-5",
-    "eu.anthropic.claude-opus-4-7",
     "eu.anthropic.claude-opus-4-8",
     "eu.anthropic.claude-haiku-4-5-20251001-v1:0"
   ]