ci: run install tests in parallel on macOS and Linux

[zimeg]

Summary

• Split install tests into a separate matrix job running in parallel with lints/unit tests
• Runs on both macos-latest and ubuntu-latest to validate install scripts on both platforms
• Removes install tests from the main lint-test job to speed up the critical path

Test plan

• Verify install-tests job runs successfully on macOS
• Verify install-tests job runs successfully on Ubuntu
• Verify lint-test job still passes without install tests step

🤖 Generated with Claude Code

Co-Authored-By: Claude svc-devxp-claude@slack-corp.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.66%. Comparing base (afcb85d) to head (8990f22).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
+ Coverage   71.64%   71.66%   +0.02%     
==========================================
  Files         225      225              
  Lines       19074    19080       +6     
==========================================
+ Hits        13665    13674       +9     
+ Misses       4202     4201       -1     
+ Partials     1207     1205       -2     

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[zimeg]

🔭 The most recent commits were experimental in timing. We find:

• Linux - Lints+Tests: 3:15 minutes
• MacOS - Lints+Tests: 2:18 minutes

[zimeg]

Adjacent change worth folding in: #652 (the Pscx Expand-Archive fix for #651) restructures this same .github/workflows/tests.yml — it pulls install tests out of the lint-test job into a dedicated install-tests matrix, which is exactly the split this PR proposes, and it already covers macos-latest + ubuntu-latest plus a windows-latest leg. So the CI-matrix half of this PR is largely subsumed once #652 lands; the still-unique parts here are the archive.sh / install.sh / install-dev.sh / .goreleaser.yml / cli_autoupdate.go changes, which #652 doesn't touch.

Suggest rebasing this on top of #652 and narrowing it to the archive/install-script + autoupdate changes, so the two don't clobber each other in tests.yml. Happy to help reconcile.