fix(main): exit non-zero when Cobra returns an error

[stantheman0128]

Summary

When a Cobra subcommand returned an error, main logged it but the process still exited 0. That broke shell scripts and CI that rely on $? after zeabur ... failures (for example invalid flags or missing required args).

Refactor to run() int and os.Exit(run()) so any Execute() error returns exit code 1. Subcommands that call os.Exit with a remote command status (service exec, server exec) are unchanged.

Related to #268 (npm wrapper exit propagation); this fixes the Go entrypoint path.

AI disclosure

AI-assisted patch; human-reviewed by Stan Shih (stantheman0128) before publish.

Verification / Evidence

go test ./cmd/... -v -count=1
# TestRun_UnknownSubcommandReturnsNonZero PASS (run() non-zero)
# TestRun_VersionReturnsZero PASS (run() == 0)

go test ./... -count=1   # green

Before: go run ./cmd/main.go this-subcommand-does-not-exist exited 0 on Windows 11.
After: same invocation exits 1.

What was not tested

• Live npx zeabur end-to-end against the published npm tarball (local go run only).

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