[Feature] mantle upgrade command and install script with upgrade detection

[michaelmcnees]

Problem

There is no way to upgrade Mantle from the CLI. Users must manually download new binaries or re-run go install. There is also no proper install script — only a quickstart.sh that builds from source.

Proposed Solution

1. mantle upgrade CLI command

mantle upgrade          # Check for update, prompt to install
mantle upgrade --check  # Check only, print available version
mantle upgrade --yes    # Upgrade without prompting

Behavior:

• Query GitHub Releases API for latest version
• Compare with current mantle version
• Download the correct binary for runtime.GOOS/runtime.GOARCH
• Verify checksum against checksums.txt from the release
• Replace the current binary in-place (or prompt for sudo if needed)
• Skip if already on latest version

2. Install script (install.sh)

A curl-pipe-bash install script hosted on the docs site:

curl -fsSL https://mantle.dev/install.sh | bash

Behavior:

• Detect OS and architecture
• Check if mantle is already on PATH
  • If yes: show current vs latest version, prompt to upgrade
  • If no: download latest, install to /usr/local/bin (or ~/.local/bin)
• Download binary from GitHub Releases
• Verify checksum
• Make executable

3. Relates to

• [Feature] Additional Installation Methods #11 — Additional Installation Methods
• [Feature] Install Script #46 — Install Script

[michaelmcnees]

Shipped in this milestone: mantle upgrade CLI command (--check, --yes, container detection, atomic binary replace with cross-device fallback, sudo hint on permission error) and install.sh curl-pipe-bash bootstrapper. Both handle version normalization between goreleaser's bare semver output and the GitHub API's v-prefixed tag names. Closes #46 and #76.