feat(chat): add fast mode control

[janburzinski]

Summary

• expose provider fast-mode config through the ACP session model and runtime
• add a Fast mode selector to the chat composer model menu
• cover config derivation and provider config updates with focused tests

Why

Codex and Claude expose fast mode as provider-owned ACP config options, but Emdash previously ignored those categories, so users could not view or change the setting from chat.

Demo

https://cap.link/mam1dvkjp6yz93v

[greptile-apps]

Greptile Summary

This PR adds fast-mode control to ACP chat sessions. The main changes are:

• Derive provider fast-mode options from ACP configuration updates.
• Route fast-mode changes through the session runtime.
• Add a fast-mode selector to the chat composer.
• Preserve unrelated config groups during partial updates.
• Add focused runtime, reducer, fixture, and UI coverage.

Confidence Score: 5/5

This looks safe to merge.

• Partial config updates now preserve unrelated selector state.
• Fast-mode values and provider config IDs remain consistent across the core, runtime, renderer, and UI layers.
• No blocking issue was found in the updated code.

Important Files Changed

Filename	Overview
packages/core/src/acp/reducer/config-derive.ts	Adds fast-mode derivation and preserves config groups omitted from partial updates.
packages/runtime/src/acp-agents/session/cell.ts	Maps fast-mode changes to the provider-owned config option ID.
apps/emdash-desktop/src/renderer/features/conversations/acp/acp-chat-store.ts	Exposes fast-mode state and updates to the renderer.
packages/ui/src/react/components/chat-composer/index.tsx	Adds the fast-mode selector to the model menu.

_{Reviews (2): Last reviewed commit: "fix(acp): preserve config groups on part..." | Re-trigger Greptile}

[janburzinski]

@greptileai