Add bns exec command for arbitrary command execution in containers

[alexo-bunnyshell]

Summary

This PR implements a new top-level bns exec utility command that allows running arbitrary commands in component containers, similar to kubectl exec and docker exec.

Command Structure

• Positional argument: bns exec <component-id> instead of bns exec --id <id>
• Context fallback: Works without component ID if set in context
• Top-level utility: Registered alongside git, port-forward, and other utilities

Key Features

• Execute arbitrary commands in component containers
• Interactive or explicit pod/container selection via wizards
• TTY and stdin control via --tty and --stdin flags (no shorthands to avoid global -t timeout flag conflict)
• Defaults to interactive shell (/bin/sh) when no command specified
• Auto-enables --tty and --stdin for interactive shells
• Supports namespace/pod-name format for pod specification
• Reuses existing K8s exec infrastructure (pkg/k8s/kubectl/exec)

Usage Examples

# Interactive shell (auto-enables --tty --stdin)
bns exec comp-123

# Run command
bns exec comp-123 -- ls -la /app

# With explicit flags
bns exec comp-123 --tty --stdin --pod my-pod -c api -- /bin/bash

# From context (no ID needed)
bns configure set-context --component comp-123
bns exec --tty --stdin

# Pipe local file to remote container
bns exec comp-123 --stdin -- python3 < script.py

Implementation

• New directory: cmd/exec/
• New file: cmd/exec/root.go (245 lines)
• Modified: cmd/utils/root.go (registered exec command)
• Pattern: Follows other utility commands (git, port-forward)