autoupdate 3/3: Apply pending update and re-exec

[yahanxing-stripe]

Summary

• On startup, if a marker file exists, download the new binary, verify checksum, atomically replace the current binary, and re-exec the process
• User sees:

  Automatically updating Stripe CLI from 1.23.0 to 1.23.1.
  To disable auto-update, set STRIPE_NO_AUTO_UPDATE=1 or add auto_update = false to ~/.config/stripe/config.toml
  Updated successfully ✓
  

• If download or checksum fails: prints a one-line warning, clears marker, continues with current version
• Binary is never left in a broken state (download to temp → atomic rename)
• Supports both tar.gz (macOS/Linux) and zip (Windows) archives

This is part 3/3 of the auto-update feature for curl-installed binaries.
Stack: #1767 (config) → #1768 (checker) → this

Test plan

• Manual test: build with an old version string, place in ~/.stripe/bin/, run a command, verify marker is written
• Manual test: with marker present, run again — verify download + re-exec
• Verify no-op for Homebrew-installed binary
• Verify opt-out via env var and config.toml both skip the update

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