[FEATURE] Cross-session conversation channel: let two users' Claude Code sessions talk to each other

Summary

There's no first-class way for two separate Claude Code sessions — belonging to two different people — to exchange messages. Not sharing one session (that's #60082), but a channel where my session and your session can talk: hand off work, ask each other for input, or coordinate an async workflow, with the other human's approval.

How this differs from #60082

• #60082 = multiple humans collaborating live inside one shared session (Google-Docs style).
• This = each person keeps their own session, and the sessions message each other (a cross-session mailbox / conversation), gated by per-message approval. Complementary, not a duplicate.

Current state

• SendMessage / peer primitives exist, but they assume a single account (my own local/cloud sessions), not two different humans.
• Remote Control is single-user (me driving my own sessions from another device).
• claude.ai "share" links are read-only.
• The only cross-session channel today is a shared file both agents read/write — I used exactly this pattern today to coordinate two of my own sessions, and it works, but it's a manual workaround with no addressing, no approval model, and no cross-account support.

Use cases

• Async handoff across a team: session A finishes a slice and hands the context to teammate B's session directly, richer than pasting a transcript.
• Enterprise coordination: in a large org, work moves across people constantly; letting their agents exchange messages (with approval) fits how teams actually operate.
• Orchestration / long-running tasks: a teammate's session can nudge or query my overnight agent without taking over my session (the observer-session pattern, but first-class).

Proposed shape

• A cross-account mailbox: address another user's session, send a message, they approve receipt (permission scopes: notify / suggest / act).
• Token-based, expiring, opt-in — the recipient always consents per message or per channel.

Prior art (demand signal)

People are already building this outside Claude Code, which suggests real demand:

• SwarmDo/swarmdo#44 — "Cross-session agent mailbox (multiplayer swarms)".
• currentbits/solenta#370 — "Multiplayer threads: a second human can join and steer a live thread".
• repowire and mobsession.ai — third-party relays for cross-account session sharing/steering (mentioned in #60082).

Why it matters

Claude Code is increasingly used inside teams, but the paradigm is still "single user + agentic delegation." Cross-session conversation is the async, team-shaped complement to #60082's live-shared-session ask.