Add Shell Completion Support

[jtdub]

Problem

Users must remember all command names and option flags. Shell completion would improve the CLI experience significantly.

Proposed Solution

Add shell completion using Click's built-in completion support for bash, zsh, and fish.

What Should Complete

• Command names: remediation, rollback, future, list-platforms, version
• Platform names: ios, nxos, junos, etc.
• File paths for --running-config and --generated-config
• Output formats: text, json, yaml

Acceptance Criteria

• Bash completion works
• Zsh completion works
• Fish completion works (nice to have)
• Platform names complete
• File paths complete
• Installation instructions documented