feat(completion): add bash, zsh, fish completions and completion subcommand

[vollate]

Summary

• Add rich shell completion scripts for bash, zsh, and fish with dynamic password store entry listing (respects $PASSWORD_STORE_DIR and -R/--repo)
• Implement pars completion {install|uninstall|generate} subcommand for one-command setup with shell auto-detection
• Remove unused clap_complete build-dependency, add dirs crate for cross-platform path resolution

Test plan

• cargo build compiles cleanly
• pars completion generate --shell bash outputs bash completion script
• pars completion generate --shell zsh outputs zsh completion script
• pars completion generate --shell fish outputs fish completion script
• pars completion generate --shell powershell outputs PowerShell script
• pars completion install auto-detects shell and installs
• pars completion uninstall removes installed completions
• Test tab completion in bash/zsh/fish after install

🤖 Generated with Claude Code