add a setting to control whether `*.instructions` are added or not

[aeschli]

Allow users to disable '*.instructions' with one setting

[aeschli]

related: https://github.com/microsoft/vscode/issues/267596

[roblourens]

And AGENTS.md, maybe for inspiration, like omitBaseAgentInstructions

[aeschli]

There's already a setting for AGENT.md: chat.useAgentsMdFile and chat.useNestedAgentsMdFiles

[OrenMe]

This is kind of like "private" chat - u want to be able to run a request with clean context. I would also want this, maybe add it as long or right click press when sending the user message. This would also be great if we can compose this in custom agent files - I might want different agents with different personas and today we can already choose tools, why not allow to choose instructions? Would also work great in frontmatter with tree widget that can enumerate the available instructions and allow to choose Also this can be applied to any sub agent that will be triggered from the man custom agent - if no custom agent is defined for the sub agent so copy the current agent available instructions or if a custom agent is defined for the sub agent so allow to use any defined ones it has