[PR Preview] Write stakeholder install guide (sshuttle, sideload, simulator, emulator)

[piyalbasu]

Goal

Write a stakeholder-facing install guide covering every Phase 2 preview channel: extension sideload, iOS Simulator .app install (macOS), Android emulator APK install, and sshuttle setup (required to reach sandbox-backed builds).

Acceptance criteria

• A non-engineer can follow the guide and install builds on Chrome (extension), iOS Simulator (macOS), and Android Emulator without asking questions
• sshuttle setup section recommends the /preview Claude Code skill as the primary path; manual sshuttle install is documented as a fallback
• Troubleshooting section covers: "I can't reach the sandbox URL" (likely sshuttle not running), "I can't find the draft release" (wrong permissions / not in stellar org), "I don't have a Mac" (physical-device testing is deferred to Phase 3 — no path today; document this)
• Lives in a place stakeholders can find — e.g., a Notion page or docs/PREVIEW_INSTALL.md in stellar/freighter
• Fresh-profile reviewer protocol is called out (per design doc § Internal Review Hygiene): preview installs should use a clean browser profile / fresh test wallet, never real keys
• Notes that physical-device iOS/Android testing returns in Phase 3 (Tailscale + TestFlight/Closed Testing) — not available today

Dependencies

• Blocks on [PR Preview] Integrate extension workflow with freighter-config (sandbox URLs) #2766 (extension workflow integration), [PR Preview] Build iOS Simulator .app for PR preview (Phase 1) freighter-mobile#860 (iOS Simulator), [PR Preview] Build Android Emulator APK for PR preview (Phase 1) freighter-mobile#861 (Android emulator) — the guide needs stable build artifact shapes to document accurately

Reference

Fullstack PR Preview Flow design doc — § Security → Internal Review Hygiene, § Phase 2 rollout