[kvnloo]

Closes #72. Fork prototype; upstream venue upstream discussion 30355.

Problem

On Android there was no way to hand a session off to a new thread or to share it with a friend:
the thread's ⋮ menu had neither, and typing /handoff showed no popup. Mobile could only manage a
guest link someone had already created on web.

What existed (upstream main 550c5fa34b)

• Guest links (Labs beta, needs T3 Connect): serverEnvironment.createGuestShare
  (packages/client-runtime/src/state/server.ts), created from web's sidebar/thread menu and command
  palette. Mobile only had ThreadGuestShareBar (stop sharing; send/dismiss guest messages).
• thread.fork with sourcePoint: latest_stable (Orchestrator.ts latestStableRun) already
  forks a thread at its latest completed, checkpointed run into a listed thread; forkSideChat uses
  it with sideChat: true. Cross-provider context moves automatically.
• No /handoff anywhere. Mobile slash menu: /model /plan /default, then provider commands.

Change

No server command or contract change; client only.

• Shared rules (packages/shared/src/threadHandoff.ts): /handoff [focus] as the whole
  message; new thread title <title> (handoff); available once a reply has finished and nothing is
  running (mirrors the server's latest_stable); the provider-collision rule.
• Client op threadEnvironment.forkForHandoff: thread.fork at latest_stable without
  sideChat.
• /handoff [focus] in web and mobile composers: forks, waits for the new thread, sends the
  focus text there as its first message (with the new thread's own modes), and opens it. The menu
  item shows why it is unavailable when it cannot run; attachments/context are refused with a
  message.
• A provider's own /handoff stays reachable: T3's built-in is listed first; the provider's is
  listed after it with its own description (or "The provider's own command"). Picking the
  provider's item from the menu sends the text to the provider; typing /handoff or picking T3's
  runs the handoff.
• Mobile thread menus: Android header overflow (⋮), iOS header Share menu and the command
  palette offer Hand off to new thread, Share with guest (→ New guest link with a confirm
  while one is active) and Stop sharing. Creating a link copies it and opens the native share
  sheet. GuestShareCreateError reasons map to copy that says where to fix it (guest links off →
  Settings → Labs → Guest links on the machine running T3 Code; no T3 Connect → start it there).
• Docs: docs/user/guest-links.md (mobile creates links), docs/user/composer.md (Hand off to a new
  thread).

Entry points added

Surface	Entry points
Web / desktop	composer /handoff [focus]
Android	thread ⋮ overflow: Hand off to new thread, Share with guest / New guest link, Stop sharing; composer /handoff [focus]; command palette (same three)
iOS / iPadOS	thread header Share menu (same three); composer /handoff [focus]; command palette

Verification

Run on the exact head f7ea30a09a in a clean tarball checkout on a fleet runner
(kvnloo/fleet-runner run 41719040164, avrea-ubuntu-latest-8-vcpu, Node 26.12.1, pnpm 12.14.0,
pnpm install --frozen-lockfile). groot could not install: the OOM guard killed pnpm install
three times.

vp test run src/threadHandoff.test.ts                    (packages/shared)   Tests  9 passed (9)
vp test run threadShareMenu.test.ts use-composer-command-menu.test.ts
                                                         (apps/mobile)       Tests 18 passed (18)
vp test run composerSlashCommandSearch.test.ts composerMenuHighlight.test.ts
                                                         (apps/web)          Tests  6 passed (6)
tsc --noEmit --singleThreaded   packages/shared, packages/client-runtime, apps/mobile, apps/web: exit 0
vp fmt --check <26 touched files>                                           exit 0
vp lint --report-unused-disable-directives <24 touched ts/tsx>              0 warnings, 0 errors
pnpm run knip:check                                                         exit 0

Negative controls in the same run (tests must bite):

• Removing the picked-provider check from resolveHandoffSend → threadHandoff.test.ts fails
  "the provider's picked /handoff goes to the provider" (expected { focus: 'tests' } to be null).
• Ranking T3 slash commands after provider ones → composerSlashCommandSearch.test.ts fails
  "lists T3's /handoff first and keeps the provider's /handoff reachable" (order flipped).

Not verified

• No device or browser run yet: no Android/iOS/web dogfood, screenshots or video.
• A live server round-trip of forkForHandoff (it reuses the thread.fork path the server tests
  cover; no server code changed, so no server test was added).
• iOS native header menu rendering (type-checked only).
• Web thread action menu / web command palette "Hand off" entries are not added: web's handoff guard
  needs the thread's runs, which only the chat view holds.

Commit map

1. d4ff6d5670 feat(client-runtime): fork a thread at its latest reply for /handoff — shared rules +
   tests, forkForHandoff.
2. 353ee0272f feat(mobile): share with guest and hand off from the thread menus — action builder +
   tests, useThreadShareActions, Android overflow, iOS Share menu, command palette.
3. 96ef9399fd feat(mobile): /handoff [focus] in the composer — slash item, collision rule, send.
4. 0cc4192f24 feat(web): /handoff [focus] in the composer — slash item, collision rule, send,
   useThreadHandoff.
5. f7ea30a09a docs(user): mobile guest links and /handoff.

Model and harness: Claude Opus 5.5 via Hermes ACP in T3 Code.