docs: add the agmsgd RFC for comment

[fujibee]

Publishes the agmsgd design for comment, ahead of implementation, as docs/design/agmsgd-rfc.md (with a Japanese translation alongside).

What it proposes

• One resident daemon replacing the per-session watcher processes, the per-team sync engines, and the Codex helper stack.
• noticed split from read — a notification can never consume a message; only an actual inbox fetch marks anything read.
• A one-line notification instead of pushed full text, so an agent mid-task spends context on a message only when it decides to, with search (Gmail-style operators over subject/summary/body) as the other half of that trade.
• Subscription carries identity, which makes claiming a role explicitly unnecessary: a session subscribes at startup from what it already knows, and the subscribed name is locked by default.
• Control messages executed by the daemon, mechanically and by kind, instead of delivered to the target model to comply with.
• Codex without the launch wrapper — hooks plus a poke into a named pane, neither of which changes how Codex starts.

Review focus

This is prose, so the useful review is factual rather than structural:

• Anything asserted about current behavior that is wrong. Every figure and mechanism in it was measured rather than recalled, but measurements can still target the wrong thing.
• Anything promised that the design does not actually deliver, or that a later ruling already reversed.
• Places where a reader who does not already know the internals would be lost.

The six open questions at the end are the ones intended for outside readers once this is published for discussion; they are not blockers for merging the document.

Not in scope here

It is an RFC, not a decision record — settled decisions continue to land as ADRs under docs/adr/. Nothing in it changes code.