Add bns exec command for arbitrary command execution in containers

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:
- Component ID as positional argument: bns exec <component-id>
- Falls back to context if component ID not provided
- Registered as top-level utility command (not under 'bns components')

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)
- 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)

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)

Usage Examples:
- Interactive shell: bns exec comp-123
- Run command: bns exec comp-123 -- ls -la /app
- With flags: bns exec comp-123 --tty --stdin --pod my-pod -c api
- From context: bns configure set-context --component comp-123; bns exec
- Pipe input: bns exec comp-123 --stdin -- python3 < script.py

Documentation:
- Updated CLAUDE.md with comprehensive exec command documentation
- Added "Utility Commands" section with usage examples
- Updated workflow examples to use new command syntax
- Documented positional argument pattern and flag conflicts
- Added design notes about global flag conflicts

Design Decisions:
- Positional component ID instead of --id flag (cleaner UX)
- No shorthand flags to avoid conflict with global -t (timeout) flag
- Follows docker/kubectl conventions for TTY and stdin handling
- Auto-detection of interactive mode when no command specified

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: alexo-bunnyshell