RFC: share a chat — /handoff to a new thread and collab guest links, Android first #72

RFC for the owner's "share a chat" ask. UX wave: #57 · Restaurant RFC: #66

Problem

I want to (a) hand a session off to a new session, and (b) share a session with friends so we work in it together, like oh-my-pi's /handoff, /collab and /share. It has to work from the Android app. Today on Android neither the thread's 3-dot menu nor /handoff in the composer offers anything.

What already exists (upstream main 550c5fa34b; live install 0.1.7-nightly.20261103.4119)

oh-my-pi for reference: /collab (control link) and /collab view (read-only) over a content-blind relay with the key in the URL fragment, many participants, joinable from a terminal or browser; /share uploads a redacted, encrypted snapshot; /handoff [focus] writes a work-focused brief (now compacts in place; earlier versions started a fresh session).

Plan (smallest first)

1. Android parity + /handoff (prototype in progress, fork PR to follow)
   • Thread overflow (Android) and header options (iOS): Share with guest → existing createGuestShare → native share sheet / copy; clear messages for disabled and connect_unavailable.
   • /handoff [focus] built-in on mobile and web composers, plus Hand off to new thread in the mobile overflow: thread.fork at latest_stable into a new top-level thread, open it, send the focus text as its first message. No new contract.
2. View-only guest link: a link that streams the thread but has no composer. Smallest step toward collab roles.
3. Collab roles (viewer / collaborator / owner, several people, presence, driver handoff): upstream Ideas 27708.
4. Static share/export of a finished thread (Markdown/JSON, secrets redacted): upstream Ideas 11820.
5. Hand off to another machine (fork into a thread on a different environment).

Experiments

• E1: scoped unit tests for slash-item building, menu builders and error mapping (RED on 550c5fa34b, GREEN on the branch).
• E2: Android, real device: preview-variant APK built on fleet-runner from the PR head, against a sandbox server with guest links enabled. Steps: /handoff fix the tests from a thread with replies → new thread opens with the inherited history and runs the focus message; 3-dot → Share with guest → share sheet shows a URL; open it in a browser, send a guest message, approve it on the phone; Stop sharing ends the guest stream. Screenshots + statev2.sqlite rows (thread.fork events, guest share state) as evidence.
• E3: fork context transfer cost: tokens carried by /handoff into a same-provider native fork vs a cross-provider portable handoff on one real long thread (read-only snapshot of real data).

Upstream venues

• New Ideas post for Android parity + /handoff + view-only link (link to follow).
• Collab roles: 27708 (open, 5 votes). Related closed duplicate: 17196.
• Export/static share: 11820.

Acceptance

• Android: 3-dot → Share with guest creates a link and opens the share sheet; errors are readable
• Android + web: /handoff appears in the slash popup and creates and opens the handoff thread
• Android: 3-dot → Hand off to new thread
• Docs updated (docs/user/guest-links.md, handoff section)
• E1–E2 evidence attached; E3 numbers recorded
• Upstream Ideas post linked here