feat: add commands config for enabling/disabling slash commands

[Tarquinen]

Summary

• Add commands section to plugin config allowing users to enable/disable /dcp-context and /dcp-stats commands
• Commands check their enabled status before executing and log when disabled
• Update README with commands documentation section
• Fix spacing in dcp-context UI output (extra space after % signs)

Config Structure

"commands": {
    "context": {
        "enabled": true
    },
    "stats": {
        "enabled": true
    }
}