fix(install): support normalized dev deb assets

[jtoelke2]

Summary

Fix install-dev.sh so the dev Debian installer can find the normalized dev release assets published by Release Dev.

Related Issue

Related to OS-49.

Changes

• Prefer openshell-dev-<arch>.deb entries from openshell-checksums-sha256.txt.
• Keep the previous openshell_..._<arch>.deb lookup as a fallback for compatibility.
• Add a focused POSIX shell lookup test for normalized names, binary checksum markers, legacy fallback, preference order, and architecture mismatch.
• Include install-dev.sh in the install-script workflow path filters and run the new lookup test there.

Testing

• sh -n install-dev.sh
• sh -n e2e/install/install_dev_lookup_test.sh
• sh e2e/install/install_dev_lookup_test.sh
• git diff --check
• mise run pre-commit passes

mise run pre-commit was attempted locally. It did not pass due to unrelated existing workspace issues: markdownlint picked up untracked .codex-learning/*.md files, and test:rust failed in ssh::tests::launch_editor_returns_friendly_error_when_binary_missing. The targeted installer checks passed.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)