feat: add Claude Opus 4.8 model definition

[skxie]

Summary

• Add the static Claude model definition for claude-opus-4-8.
• Mirror the claude-opus-4-7 capabilities: 1M context, 128k max completion tokens, and the same thinking level support.

Verification

• Built local Docker image clirelay:claude-opus-4-8-7f969176.
• Deployed with docker compose up -d using the local image.
• Confirmed cli-proxy-api and clirelay-updater started successfully with commit 7f969176.
• Confirmed the built binary contains claude-opus-4-8 / Claude Opus 4.8.
• User verified the new model is accessible from Claude CLI.
• Ran gofmt on internal/registry/model_definitions_static_data.go.
• Ran git diff --check.
• Ran go test ./internal/registry.
• Ran go test ./cmd/server.
• Ran go test ./internal/registry ./internal/thinking/....

Notes

go test ./... currently has unrelated failures in cmd/updater and internal/usage around existing permission/mode expectations; the registry package passed.