fix(upgrade): run script installer pipeline via shell

[zerone0x]

Summary

Fixes #281

gortex upgrade --run now executes the installer-script command through sh -c, so the documented installer pipeline is interpreted by a shell instead of being split into raw curl arguments.

Changes

• Add upgradeExecCommand to choose shell execution only for commands with shell metacharacters
• Keep plain argv execution for commands such as go install ...
• Add regression coverage for the script pipeline and plain go install path

Validation

• gofmt -w cmd/gortex/upgrade.go cmd/gortex/upgrade_test.go — blocked: this runner does not have gofmt/Go installed
• Static diff review only in this environment; change is limited to fixed upgrade command templates and tests

Test failed

[zzet]

Thanks for this @zerone0x — your diagnosis and the sh -c approach were spot on. I've carried your intent and test forward in #316 (you're credited as co-author on the commit).

Two small corrections were needed relative to this PR's test, which its notes mention was never run (no Go in that environment):

• It asserted on cmd.Path, but exec.Command resolves a bare name via LookPath, so Path becomes /bin/sh and /usr/local/go/bin/go once those are on PATH — the assertion would fail in CI. feat(cli): upgrade/uninstall self-maintenance (fixes #281, refs #298) #316 asserts on cmd.Args[0] instead.
• It passed rootCmd, whose Context() is nil outside cobra's Execute(), so exec.CommandContext(nil, …) panics. feat(cli): upgrade/uninstall self-maintenance (fixes #281, refs #298) #316 has the helper take a context.Context directly, which removes that foot-gun and keeps it unit-testable.

#316 supersedes this PR — closing in favour of it. Really appreciate the fix. 🙏