Installer follow-ups: curl script and completion UX

[rmanibus]

Context

PRs #114 and #115 introduced:

• curl installer with checksum verification
• optional shell completion installation

Those features are merged and working, but we should track a focused follow-up pass for robustness and UX polish.

Linked PRs

• feat: add curl installer with checksum verification #114
• feat: support completion setup in curl installer #115

Follow-up scope

• Add automated coverage for scripts/install.sh behavior (option parsing, shell detection, path handling) using a script-test harness.
• Verify completion install paths and activation hints across bash/zsh/fish on macOS and Linux CI runners.
• Improve failure guidance when install directory is not writable and when completion target directories are missing from shell config.
• Decide whether completion install should support all shells in one run (vs one selected shell).
• Add explicit docs for uninstall/cleanup of completion files created by the installer.
• Evaluate a --print-completion-path and/or --dry-run mode for safer automation.

Definition of done

• Installer/completion behavior is covered by automated tests.
• Documentation includes install + completion + cleanup workflows.
• Known UX edge-cases are handled with actionable messages.