[FEATURE] Allow multiple slash commands on a single line

Problem Statement

/model (and similar commands) consume the rest of the input line as their argument. Typing:

/model opus /effort xhigh

results in "Model 'opus /effort xhigh' not found" instead of running both commands.

Proposed Solution

Let / delimit a new recognized slash command mid-line, so:

/model opus /effort xhigh /compact

executes sequentially, equivalent to entering each on its own line. Backwards compatible: a command keeps consuming args until the next token matches a known slash command name.

Motivation

• Quickly configure a session
• Copy/paste-able command presets
• Compose multiple settings in one line
• Easier to document/automate common configurations