[codex] Remove unused remote rootfs build

[EYHN]

Summary

• Remove the unused remote rootfs Docker context and build script.
• Drop the stale remote-rootfs Makefile target so SandboxVM only exposes maintained local build targets.
• Remove git from the local SandboxVM rootfs package list.
• Remove erofs-utils from the local SandboxVM rootfs package list; EROFS image creation remains a build-host concern.
• Keep net-tools and e2fsprogs in the local rootfs for runtime diagnostics and overlay formatting.
• Remove the local VM SSH server/debug-access flow while preserving the SSH client package for outbound SSH usage.
• Build the guest vmd binary with -trimpath -ldflags="-s -w" to strip debug metadata from the local rootfs init binary.
• Remove unused socat and udhcpc packages now that the VM no longer starts an SSH forwarder or DHCP client.
• Prune runtime rootfs cache/test/doc residue: Python __pycache__, Python stdlib tests, package docs/man/info/lintian data, debconf cache, and logs.
• Remove unused AppIconCOOKIE and AppIconLIQUID assets, and narrow Release/Staging alternate app icons to the two icons exposed by product code.
• Exclude Debug/TestFlight icon sources from production Release assets; Staging keeps TestFlight as its primary icon but excludes Debug-only icon sources.

Validation

• make -C sandbox-vm -n framework
• make -C sandbox-vm -n vm
• make -C sandbox-vm proto
• cd sandbox-vm && go test ./cmd/vmd/... ./sdk/apple/... ./internal/... ./pkg/...
• xcodebuild -workspace OpenBridge.xcworkspace -scheme OpenBridge -configuration Release -showBuildSettings confirms ARCHS = arm64 and Release alternate icons are AppIconBLUEBLOCK AppIconMERCURY.
• xcodebuild -workspace OpenBridge.xcworkspace -scheme OpenBridge -configuration Staging -showBuildSettings confirms Staging alternate icons are AppIconBLUEBLOCK AppIconMERCURY.
• cd macos && BUILD_CONFIGURATION=UnsignedDebug bash DevKit/Scripts/workspace_build_debug.sh
• cd macos && xcodebuild -workspace OpenBridge.xcworkspace -scheme OpenBridge -configuration Release -destination platform=macOS -derivedDataPath .build/DerivedDataReleaseAudit -skipPackagePluginValidation -skipMacroValidation CODE_SIGNING_ALLOWED=NO build
• cd macos && xcodebuild -workspace OpenBridge.xcworkspace -scheme OpenBridge -configuration Staging -destination platform=macOS -derivedDataPath .build/DerivedDataReleaseAudit -skipPackagePluginValidation -skipMacroValidation CODE_SIGNING_ALLOWED=NO build
• Built a temporary rootfs and verified sshd is absent while ssh, ssh-keygen, ifconfig, netstat, route, mkfs.ext4, python3, rg, curl, ip, ping, jq, fdfind, and file remain available.
• Built a temporary stripped-vmd rootfs and verified /sbin/init is stripped.
• Temporary rootfs size after vmd stripping: 69M compressed EROFS, 210M exported tar.
• Built a temporary rootfs after removing socat and udhcpc; verified both are absent and required runtime tools remain available. Same-compressor comparison: lzma EROFS 65,294,336 -> 64,598,016 bytes; exported tar 220,562,432 -> 218,688,000 bytes.
• Built a temporary rootfs after pruning cache/test/doc residue; verified Python imports, python3 -m pip, and python3 -m venv still work. Same-compressor comparison from previous step: lzma EROFS 64,598,016 -> 55,078,912 bytes; exported tar 218,688,000 -> 196,113,408 bytes.
• Inspected rebuilt Release Assets.car: AppIcon, AppIconBLUEBLOCK, and AppIconMERCURY are present; AppIconDev, AppIconTESTFLIGHT, AppIconCOOKIE, and AppIconLIQUID are absent. Release Assets.car is 16M in the local unsigned build.
• Inspected rebuilt Staging Assets.car: AppIconTESTFLIGHT, AppIconBLUEBLOCK, and AppIconMERCURY are present; AppIconDev, AppIconCOOKIE, and AppIconLIQUID are absent. Staging Assets.car is 15M in the local unsigned build.