Shared Sessions v0 (slice 9): pan share and pan join CLI #4480

PRD: .pan/drafts/pan-658.md (rev 3), section Clients & join paths → CLI

Part of PAN-658 Shared Sessions v0, slice 9. PAN-658 itself delivers slice 1: the Cloudflare share service (services/share/, a Worker plus one Durable Object per room) and the signaling protocol types in @overdeck/contracts. This slice builds on it.

What

• pan share <conversationId> prints the link and the join code.
• pan join <link-or-code> signs in through the PAN-4330 device flow if needed, starts the local dashboard if it is not running, posts the join intent, and opens the shared route.
• Zero-install join (operator requirement, 2026-10-02): npx @overdeck/core join <link-or-code> works on a machine that has never installed Overdeck (Node 22+, empty OVERDECK_HOME). npx already runs the published CLI without a global install (verified: npx -y @overdeck/core --version prints 0.64.0), so the work is making join succeed cold from the npx cache: first-run setup without projects, the dashboard started from the npx package directory, and no Herdr or tmux needed for a viewer.
• The share Worker invite page (services/share/src/pages.ts invitePage) shows npx @overdeck/core join <CODE> as the no-install command, next to pan join <CODE> for installed users.
• Wrapper skills pan-share and pan-join land in the same commit as the verbs (scripts/lint-skills.sh).

Depends on

PAN-4475 (viewer surface) and PAN-4330.

Acceptance (sketch)

• The device-code poll respects slow_down (fake timers).
• pan join starts the dashboard when it is down.
• The user lands in the lobby.
• Given a machine with Node 22 and no Overdeck (no global pan, empty OVERDECK_HOME), when the user runs npx @overdeck/core join <code>, then the local dashboard starts and the user lands in the lobby.
• GET /s/:code for an active room shows both pan join <CODE> and npx @overdeck/core join <CODE>.