Fix Linux completion paths in install script

[cyq1017]

Fixes #345

Summary

• preserve the existing macOS completion destinations while using XDG-backed Linux completion directories for bash and zsh
• print mkdir -p commands before each completion redirection so the suggested commands do not fail on missing directories
• add focused shell assertions for completion paths, completion directories, and instruction output

Verification

• bash -n install.sh
• bash -n scripts/test-install-completion-paths.sh
• ./scripts/test-install-completion-paths.sh
• git diff --check

[awdemos]

Endorsed for merge.

• Clean, focused fix for [Bug]: Completions suggested by the install script don't work on Linux #345.
• Uses XDG-backed completion directories on Linux while preserving macOS paths.
• Adds mkdir -p instructions and a focused test script covering Linux, macOS, and custom XDG_* values.
• Verified locally: bash -n install.sh, bash -n scripts/test-install-completion-paths.sh, and ./scripts/test-install-completion-paths.sh all pass.

Low risk — good to land.