Refresh default model IDs for current providers

Update auto-generated opencode.json defaults to model IDs that are
valid as of April 2026:

- anthropic: claude-sonnet-4-5 -> claude-sonnet-4-6
- openai:    gpt-4o (retired Apr 3 2026) -> gpt-5.4
- bedrock:   anthropic.claude-sonnet-4-5-v1 (invalid) ->
             global.anthropic.claude-sonnet-4-5-20250929-v1:0

The Bedrock ID now uses the global inference profile (no regional
10% premium) and includes the required date stamp and :0 suffix.

.env.example

@@ -7,7 +7,7 @@
 OPENCODE_PROVIDER=anthropic
 
 # Model override (optional, defaults per provider)
-# OPENCODE_MODEL=anthropic/claude-sonnet-4-5
+# OPENCODE_MODEL=anthropic/claude-sonnet-4-6
 
 # ── API Keys (set the one matching your provider) ────────────────────
 # ANTHROPIC_API_KEY=