Clean up model specific instructions, reminders, tool enablements

[bhavyaus]

Extend prompt registry https://github.com/microsoft/vscode-copilot-chat/blob/045e2414e45bcd6c475fea99ec12f6fec59164f4/src/extension/prompts/node/agent/promptRegistry.ts#L24

to also register model specific reminders, tool enablements etc.
https://github.com/microsoft/vscode-copilot-chat/blob/917fb4d9943b68460ab5cd9d17cfc490bed89b24/src/extension/intents/node/agentIntent.ts#L69-L107

[pwang347]

Started some explorations here by supporting prompt overrides via options from the provider prompt file: microsoft/vscode-copilot-chat#1681