Add attach API for kubectl attach support

## Motivation

- Enable attaching to the main process of a running container via `kubectl attach`. Depends on the SSH proxy infrastructure.

## Required Features

1. **Attach semantics**
- Connect to main container process stdin/stdout/stderr
- No command specified - attaches to PID 1 or entrypoint
1. **Stream handling**
- Support stdin forwarding (`-i` flag)
- PTY allocation for interactive use (`-t` flag)

## Impact

- **Agent**: Low - Attach stream handling
- **Manager**: Low - Attach endpoint wrapper

## Testing Scenarios

- Attach to running container process
- stdin forwarding to attached process
- Detach without terminating process
- Behavior when main process exits

JIRA Issue: BA-3868