Generate shell completion scripts (`eko completion`)

[kavix]

🎯 Goal

Add shell completion command support (eko completion [bash|zsh|fish|powershell]) so users can enjoy TAB completion for subcommands and flags.

💡 Expected Usage

# Generate completion script for Zsh
eko completion zsh > ~/.zsh/completion/_eko

🛠️ Implementation Steps

1. Create cmd/completion.go.
2. Utilize Cobra's built-in GenBashCompletion, GenZshCompletion, GenFishCompletion, and GenPowerShellCompletion methods.
3. Document shell installation instructions in the command Long description.
4. Add unit test in cmd/commands_test.go.

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📖 See full details in GOOD_FIRST_ISSUES.md — Issue #7
🌟 Great for first-time contributors! Read CONTRIBUTING.md to get started.

[UmairAhmedImran]

Hi @kavix , I'd like to work on this!.