Decouple background updates from controller startup

[onehoon]

Summary

• Disable Velopack early auto-apply and apply an already-downloaded package only after the process becomes the primary single instance.
• Remove the blocking startup update gate so hardware and Full1902 controller startup are independent of GitHub availability.
• Start one bounded, cancellation-aware background check/download after deferred controller startup completes; keep the current Runtime alive and apply on the next safe primary startup.
• Remove the obsolete startup-gate/update-restart plumbing and update the automatic-update README contract.

Validation

• dotnet build SteamInputAddonforClaw.slnx -c Debug --no-restore: passed, 0 warnings, 0 errors
• dotnet build SteamInputAddonforClaw.slnx -c Release --no-restore: passed, 0 warnings, 0 errors
• dotnet test tests/SteamInputAddonforClaw.Tests/SteamInputAddonforClaw.Tests.csproj -c Release --no-build --no-restore: passed, 2751/2751
• git diff --check: passed

Safety and scope

• Normal secondary launches activate the existing Runtime and return before any pending-update apply check.
• Restart replacement waits for the old single-instance owner to release before pending apply is scheduled, and forwards the original arguments including --background and --restart.
• Background update failure or cancellation is fail-open and does not perform controller teardown, PID/HidHide mutation, VIIPER detach, or Runtime restart.
• Task Scheduler priority changes, phantom cleanup, and hardware validation are not included. Physical MSI Claw validation remains pending.