Feature request: make Remote account-wide by default (all local chats visible, like Codex)

## Summary
Today, Remote sessions don't automatically surface a user's chats/sessions from across their whole account. Instead, remote access currently has to be explicitly set up on a per-project basis from the local machine before that project's sessions are reachable remotely — and the toggle/setting that does this isn't obviously named, so it's easy to miss.

## Suggestion
Model Remote after how Codex (OpenAI's coding agent) handles this: the moment a user opens Remote, all of their local chats/sessions across every project on their account are already there — no per-project opt-in step, no separately named feature to discover and turn on first. Account-wide access should be the default behavior, not something that has to be configured per-repo/per-machine before it works.

## Why this matters
- Users expect Remote to continue their local work seamlessly; discovering after the fact that a given project was never remote-enabled breaks that expectation.
- A per-project activation step that has to happen locally first, with a non-obvious name, makes the feature feel opt-in and hidden rather than something that "just works" out of the box.

The issue describes Remote sessions and the existing per-project activation flow, but names no files or tests. Start by locating the Remote entry point and tracing how local sessions become remotely visible; done means all account sessions are available by default without per-project setup.

索引モデルが issue の本文から書いたものです。