Description

Add a clawfleet upgrade command that handles the full upgrade lifecycle:

1. Download latest binary from GitHub Releases (with checksum verification)
2. Replace the current binary
3. Pull the new Docker image
4. Restart the Dashboard daemon

Motivation

Currently users must re-run curl ... install.sh | sh to upgrade. This has UX issues:

• Old daemon process may not restart properly (stale PID, port conflict)
• No feedback on what changed (old version → new version)
• Users don't know when an upgrade is available

Scope

• clawfleet upgrade — upgrade to latest release
• clawfleet upgrade --version v1.0.3 — upgrade to specific version
• Show changelog summary after upgrade
• Optionally: clawfleet version --check to check if an upgrade is available without applying