refactor: update default stack image to flexinit/agent-stack

Replace oh-my-opencode-free references with the new consolidated
flexinit/agent-stack image in source code and documentation.

README.md

@@ -213,7 +213,7 @@ data: {"url":"https://john-dev.ai.flexinit.nl","status":"ready"}
 - `PORT` - HTTP server port (default: `3000`)
 - `HOST` - Bind address (default: `0.0.0.0`)
 - `STACK_DOMAIN_SUFFIX` - Domain suffix for stacks (default: `ai.flexinit.nl`)
-- `STACK_IMAGE` - Docker image for user stacks (default: `git.app.flexinit.nl/oussamadouhou/oh-my-opencode-free:latest`)
+- `STACK_IMAGE` - Docker image for user stacks (default: `git.app.flexinit.nl/flexinit/agent-stack:latest`)
 - `RESERVED_NAMES` - Comma-separated forbidden names (default: `admin,api,www,root,system,test,demo,portal`)
 
 ### Not Used in Deployment
@@ -299,7 +299,7 @@ Available MCP tools:
 - **Wildcard DNS**: `*.ai.flexinit.nl` → `144.76.116.169`
 - **Traefik**: Wildcard SSL certificate for `*.ai.flexinit.nl`
 - **Dokploy**: Running at `http://10.100.0.20:3000`
-- **OpenCode Image**: `git.app.flexinit.nl/oussamadouhou/oh-my-opencode-free:latest`
+- **OpenCode Image**: `git.app.flexinit.nl/flexinit/agent-stack:latest`
 
 ### Network Access