Add non-blocking upgrade check on CLI startup

[renardeinside]

Summary

• Adds check_upgrade_available() in upgrade.rs — fetches the latest GitHub release tag with a 200ms timeout, compares versions, and prints a dim stderr nudge if an update is available
• Calls the check at the start of run_command() for all subcommands except upgrade (avoids redundant nudge before the upgrade flow)
• Timeout/network errors are silently logged at debug level so the CLI is never blocked

Closes #128

Test plan

• cargo build -p apx-cli compiles
• cargo test -p apx-cli — all 28 tests pass
• cargo clippy -p apx-cli — zero warnings
• Manual: apx info shows nudge when a newer version exists
• Manual: apx upgrade does not show the nudge
• Manual: disconnected network — command proceeds within ~200ms with no visible error

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