feat: replace profiles with kubernetes-style secrets

Replace the confused Profile abstraction with a dedicated Secret resource
following Kubernetes conventions. Servers now have env entries with inline
values or secretRef references. Env vars are resolved and passed to
containers at startup (fixes existing gap).

- Add Secret CRUD (model, repo, service, routes, CLI commands)
- Server env: {name, value} or {name, valueFrom: {secretRef: {name, key}}}
- Add env-resolver utility shared by instance startup and config generation
- Remove all profile-related code (models, services, routes, CLI, tests)
- Update backup/restore for secrets instead of profiles
- describe secret masks values by default, --show-values to reveal

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

docs/getting-started.md

@@ -96,10 +96,12 @@ servers:
     description: Slack MCP server
     transport: STDIO
     packageName: "@anthropic/slack-mcp"
-    envTemplate:
+    env:
       - name: SLACK_TOKEN
-        description: Slack bot token
-        isSecret: true
+        valueFrom:
+          secretRef:
+            name: slack-secrets
+            key: token
 
   - name: github
     description: GitHub MCP server