Add exec API for kubectl exec support

## Motivation

- Enable running commands inside compute sessions via `kubectl exec`. Depends on the SSH proxy infrastructure.

## Required Features

1. **Exec semantics**
- Execute specified command in container
- Stream stdin/stdout/stderr
- Return exit code on completion
1. **Interactive and non-interactive modes**
- PTY allocation for interactive use (`-it` flag)
- Direct I/O for scripted commands

## Impact

- **Agent**: Low - Exec command handling
- **Manager**: Low - Exec endpoint wrapper

## Testing Scenarios

- Single command execution with exit code
- Interactive shell session
- Piped input/output for scripting
- Environment variable and working directory support

JIRA Issue: BA-3867

Start by reading the SSH proxy infrastructure and the stated Agent and Manager responsibilities. Map how kubectl exec should support command execution, streamed stdin/stdout/stderr, exit codes, PTY allocation, environment variables, and working directories. Done means the listed single-command, interactive, piped-I/O, and environment scenarios work.

Written by the indexing model from the issue text.