[Feature Request] Implement “context hooks” which allow users or MCP servers to pass context either at the start of a conversation or alongside each prompt

Possible syntax:

```
/context add [init | preprompt ]
/context add preprompt !q-personalization --list-memories
```

Where statement can be anything type-able on the prompt.
For performance we would recommend one of:
`! - execute a shell command`
`/ - execute an internal command`
`# - execute a tool with exact parameters.` <-- I don't think we have this at the moment. Pick an appropriate character

Using this syntax, further files could be included in the context, or custom context inclusions, such as environment specific data.