feat(extensions): support allowCommands option in sendUserMessage

[max-elia]

Description

Currently, any message injected by extensions via pi.sendUserMessage() has command handling and template expansion disabled (expandPromptTemplates: false). This prevents extensions from programmatically triggering slash commands (such as connection triggers or resetting sessions).

To solve this, I propose adding a new option allowCommands?: boolean to sendUserMessage(). When set to true, the expandPromptTemplates flag is enabled for that message.

I have implemented this change and created a matching PR (see #5732). Since this is an external bridge control feature (like in pi-telegram), opting in dynamically is safe and backward-compatible.

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[mitsuhiko]

This is currently intentional. See also already existing issues:

• pi.sendUserMessage("/cmd", { deliverAs: "followUp" }) from a tool does not execute slash commands #4754
• pi.sendUserMessage doesn't execute commands #2994
• pi.sendUserMessage("/cmd", { deliverAs: "followUp" }) from a tool does not execute slash commands #4392

This is a larger change that needs to be through through properly first. Since I'm not sure to which degree we have explain this yet, but the system is currrently split between prompt templates ("custom slash commands") and actual slash commands.