feat: add s390x support to install-cli.sh

[dangrondahl]

Summary

• Map the s390x machine type to the s390x release asset in install-cli.sh
• Update the unsupported-arch error message to list s390x alongside amd64 and arm64

The linux/s390x build target was added in 4acd31e, but the install script still rejected the architecture, forcing s390x users to download the tarball manually.

Test plan

• Run install-cli.sh on a linux/s390x host and verify it downloads kosli_<version>_linux_s390x.tar.gz and installs successfully
• Confirm existing amd64 / arm64 / darwin / windows paths still work

[claude]

Claude finished @dangrondahl's task in 1m 33s —— View job

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Review of PR #936: feat: add s390x support to install-cli.sh

• Run static analysis checks
• Review code changes
• Post review feedback

Static Analysis

All checks pass:

• go build ./... — clean
• go vet ./... — clean
• go mod tidy — no drift

(Expected since this PR only touches a shell script, but good to confirm nothing else drifted.)

Code Review

This PR looks good to merge. The change is minimal, correct, and follows the existing patterns exactly.

What it does right:

• The new s390x) case block at line 153–156 mirrors the structure of the amd64 and arm64 blocks — same pattern, same debug message style.
• The error message update (line 159) correctly lists all three supported architectures.
• Verified that s390x is already a GoReleaser build target (added in 4acd31e, PR feat: add linux/s390x build target to release distribution #930) and is correctly excluded from darwin and windows builds — so the install script will find a matching kosli_<version>_linux_s390x.tar.gz asset for Linux-only installs.

No issues found — no bugs, no edge cases missed, no security concerns. The uname -m value on s390x systems is literally s390x, so the single-value case match is correct (unlike amd64/arm64 which need multiple aliases).

[dangrondahl]

Tested install of v2.24.2 across all release targets:

Target	Method	Result
linux/s390x	Docker + qemu emulation, full install	install succeeded, kosli version returned v2.24.2 ✅
linux/amd64	Docker + qemu emulation, full install	install succeeded, kosli version returned v2.24.2 ✅
linux/arm64	Docker (native on Apple Silicon), full install	install succeeded, kosli version returned v2.24.2 ✅
darwin/amd64	URL HEAD check	200 ✅
darwin/arm64	URL HEAD check	200 ✅
windows/amd64	URL HEAD check	200 ✅

Darwin/Windows aren't containerisable on macOS, but this patch only touches the Linux arch-detection branch, so the other OS paths are unchanged from main — the URL checks confirm the release assets they target exist.

[dangrondahl]

Note: I considered adding s390x coverage to install-script-tests.yml, but GitHub doesn't offer s390x runners and emulating it via qemu in CI felt heavier than the marginal value — the s390x logic is a single extra case arm. Leaving it uncovered for now.