Cherry-pick from upstream

Upstream commit: a6131438e
Author: steipete (Peter Steinberger)
Tier: AUTO-PICK (alive=3, rebranded=4)

Conflict resolution

This cherry-pick required conflict resolution for rebranded paths:

• CHANGELOG.md: Kept fork's deletion (fork removed CHANGELOG.md)
• GatewayDiscoverySelectionSupport.swift: Placed upstream's version at fork's rebranded path (RemoteClaw/ instead of OpenClaw/), rebranded imports (RemoteClawDiscovery, RemoteClawConfigFile)
• TailscaleServeGatewayDiscovery.swift: Placed upstream's version at fork's rebranded path (RemoteClawDiscovery/ instead of OpenClawDiscovery/), rebranded import (RemoteClawKit)
• GatewayDiscoveryModel.swift: Kept fork's version (fork replaced scheduleTailscaleServeFallback with scheduleWideAreaFallback; upstream's changes to the old method don't apply)
• GatewayDiscoveryModelTests.swift: Kept fork's version (tests for removed code don't apply)
• TailscaleServeGatewayDiscoveryTests.swift: Placed upstream's version at fork's rebranded path, rebranded imports
• GatewayDiscoverySelectionSupportTests.swift: New file placed at fork's rebranded path, rebranded imports and env var names (REMOTECLAW_CONFIG_PATH), domain names (remoteclaw.internal.)

Closes #914 — commit 3/4