fix(remote): preserve CLI model for remote sessions

[iscekic]

Summary

• Add SDK session type/provider tracking and a shared CLI model sentinel/label for remote sessions.
• Pin a "CLI model" option in mobile and web remote composers, selected by default, so sends omit model/variant and preserve the CLI session provider.
• Keep explicit gateway model selection behavior available and covered by SDK/mobile picker tests.

Verification

• Manual end-to-end BYOK remote session verification left for reviewer: start a CLI remote session on an Anthropic BYOK model, send from phone/web with CLI model selected, verify the CLI remains on the Anthropic provider, then explicitly select a gateway model and verify it switches.

Visual Changes

N/A

Reviewer Notes

• Branch is currently behind latest main by unrelated commits; PR diff is limited to this feature.
• PLAN.md is intentionally left untracked and not included.