fix: harden curl installer edge cases

[rmanibus]

Summary

Follow-up to review concerns raised on merged PR #114 (and related installer docs from #115).

• Harden installer failure handling for non-writable install directories:
  • handle mkdir -p failure explicitly with actionable guidance
  • preserve and print real cp/chmod errors instead of always reporting "permission denied"
• Add explicit awk dependency check since the script relies on it.
• Normalize --version values so both v1.2.3 and 1.2.3 work.
• Expand docs with a documented --no-verify escape hatch.
• Improve manual pre-built binary example with explicit checksum verification and note preferring the installer.

Validation

• sh -n scripts/install.sh
• ./scripts/install.sh --help

Context

These changes address unresolved review feedback from:

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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