feat(fork): agentbox fork + /agentbox for Claude/Codex/OpenCode

[madarco]

Summary

Adds agentbox fork — snapshot the current host agent session and resume it in a fresh box, opening it in a new terminal tab (iTerm/tmux) or background. Plus a user-invocable /agentbox command installed into Claude, Codex, and OpenCode.

• agentbox fork (apps/cli/src/commands/fork.ts): thin wrapper over each agent's existing create+teleport+attach pipeline. Flags: --agent claude|codex|opencode (default claude), --session, --provider, -n, --attach-in (default tab), --carry-yes. Background fallback when no tmux/iTerm (never hijacks the calling terminal). LLM-resilience guards: refuses inside a box (AGENTBOX_RELAY_URL), tolerates empty --provider, rejects --session with --agent opencode.
• agentbox install: writes the /agentbox command into ~/.claude/skills (fork command + renamed agentbox-info reference), and — when detected — ~/.codex/prompts/agentbox.md and ~/.config/opencode/commands/agentbox.md. Idempotent via an <!-- agentbox-managed:v1 --> sentinel. agentbox claude prints a one-time install tip.
• Per-agent session pinning: Claude uses the ${CLAUDE_SESSION_ID} skill placeholder; Codex (no session-id variable) has its agent resolve the live uuid from the newest rollout file; OpenCode starts a fresh box (session teleport not implemented yet).

Fixes folded in

• iTerm new-tab spawn (terminal/host.ts): the create window/tab ... command "..." AppleScript parameter is broken on iTerm 3.7 betas (bounces to Terminal.app); switched all modes to create-then-write text.
• Cloud resume on new-tab attach (core/provider.ts, sandbox-cloud/cloud-provider.ts, _cloud-attach.ts): cloud sessions start lazily at attach, and the new-tab re-invocation of agentbox <agent> attach carried no --resume, so hetzner forks started fresh. Added a detached build option and pre-create the resumed session before the new-tab spawn.

Scope

v1 is host→box only; box→box fork is deferred (needs downloadPath from the source box + a relay endpoint). OpenCode resume is deferred (sessions live in a multi-tenant SQLite DB).

Test plan

• pnpm -w build, typecheck, lint, vitest (329 pass) green
• agentbox install writes all 4 agent files; idempotent re-run
• Docker fork resumes the Claude session in a new iTerm tab; background fallback works
• fork --agent codex resumed a real Codex session in a box; fork --agent opencode started a fresh box
• Live Hetzner fork now launches claude --resume <id> in the box (was fresh before)
• Manual: /agentbox in Claude, /prompts:agentbox in Codex, /agentbox in OpenCode

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Note

Medium Risk
New fork/install orchestration touches session teleport and terminal/cloud attach paths; cloud detached pre-start fixes resume correctness but adds attach complexity.

Overview
Adds agentbox fork to teleport the current host Claude/Codex/OpenCode session into a new sandbox (default new iTerm/tmux tab, background when no splittable terminal so the caller’s session isn’t hijacked). It delegates to each agent’s existing create + session-teleport + attach flow, with guards for in-box runs (AGENTBOX_RELAY_URL), ambiguous recent Claude sessions, and no OpenCode resume yet.

Adds agentbox install to ship bundled host skills/prompts (/agentbox fork + agentbox-info reference) into Claude, and into Codex/OpenCode when those config dirs exist, with idempotent agentbox-managed overwrite rules. agentbox claude shows a one-time tip to run install when the fork skill is missing.

Fixes cloud resume on new-tab attach: pre-creates the tmux session detached with the full launch command (claude --resume …) before the tab re-invokes attach without extra args. iTerm spawning now uses create-then-write text instead of broken command parameters on 3.7 betas.

^{Reviewed by Cursor Bugbot for commit 7e45062. Configure here.}