feat: add raw chat agent

[fyzanshaik]

Fixes #5813

Adds a chat agent that bypasses project context and tool injection, significantly reducing token usage for general queries.

Changes

• agent.ts: Added context: "none" | "full" to agent schema and registered a native chat agent with tools disabled.
• llm.ts: Modified header/provider prompt logic to skip system instructions when context is "none".
• prompt.ts: Bypassed injection of environment details and the file tree for "none" context agents.

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Notes

• Edge Case: Session history remains when switching agents; only new requests are minimized.
• Discussion: Should switching to chat mode offer to reset the session for a truly fresh start?

[rekram1-node]

Idk if we want to add this mode rn so maybe better as plugin?, I do think you are able to customize all the system message stuff via experimental plugins rn but we should prolly look into a way to override it or set it in agents better

[fyzanshaik]

@rekram1-node A plugin sounds better, I'll look into building a plugin. But a chat mode with minimal prompt injection could be a potential mode people would love to use, I use opencode quite a lot-switching to a web app for simple query requests is not a good experience. Many times I would simply want to learn-understand a specific file or invoke a function call (eg: reading the directory structure of an open source project) and get on with it.