feat: add gh CLI, kubeseal CLI, and Helm MCP sidecar

Install GitHub CLI (gh) via official APT repo and kubeseal via GitHub
Releases binary in the Dockerfile. Add mcp-helm sidecar on port 8088
for AI-assisted Helm chart browsing, with corresponding values, schema,
deployment template, and .mcp.json configuration.

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

chart/values.schema.json

@@ -270,6 +270,9 @@
             "pgtuner": {
               "$ref": "#/$defs/mcpSidecar"
             },
+            "helm": {
+              "$ref": "#/$defs/mcpSidecar"
+            },
             "playwright": {
               "$ref": "#/$defs/mcpSidecar"
             }