… and network policy (!42)

Closes NVIDIA#34

## Summary

- Add `nav ssh-proxy` name mode (`--cluster`/`--name`) and `nav sandbox ssh-config` for VS Code Remote-SSH integration
- Fix VS Code Remote-SSH platform misdetection (was defaulting to Windows/PowerShell)
- Fix sandbox network policy for VS Code server connectivity
- Fix sandbox user shell configuration

## Problem

VS Code Remote-SSH was failing to connect to Navigator sandboxes due to three issues:

1. **Platform misdetection**: VS Code sends `uname -rsv` after a `ready:` marker and expects a response within ~1ms. Forced PTY allocation in `shell_request`, interactive bash (`-i`), and leaked supervisor env vars added enough latency that VS Code timed out and defaulted to "Platform: windows", then tried `powershell`.
2. **Broken network policy**: The `vscode` entry in `dev-sandbox-policy.yaml` was at the YAML top-level instead of nested under `network_policies:` (indentation bug). The VS Code server binary also needed a glob entry.
3. **No login shell**: The sandbox user had `/usr/sbin/nologin` as its shell.

## Changes

| File | Change |
|------|--------|
| `crates/navigator-sandbox/src/ssh.rs` | Remove forced PTY in `shell_request`, remove `-i` flag from pipe exec, add `env_clear()` + explicit `SHELL`/`PATH` to both shell spawn paths |
| `deploy/docker/sandbox/Dockerfile.base` | Change sandbox user shell to `/bin/bash`, set home to `/sandbox`, add `.bashrc`/`.profile` |
| `dev-sandbox-policy.yaml` | Fix `vscode:` indentation under `network_policies:`, add `/sandbox/.vscode-server/*` binary glob |
| `crates/navigator-cli/src/main.rs` | Add `--cluster`/`--name` to `SshProxy`, add `SshConfig`/`Policy`/`Logs` subcommands (rebase merge) |
| `crates/navigator-cli/src/ssh.rs` | Update `print_ssh_config` to use `--cluster` flag |
| `examples/vscode-remote-sandbox.md` | User-facing example and docs |

## How to use

```bash
# Create a persistent sandbox
nav sandbox create --keep my-sandbox

# Generate SSH config
nav sandbox ssh-config my-sandbox >> ~/.ssh/config

# Open VS Code
code --remote ssh-remote+nav-my-sandbox /sandbox
```

## Testing

- All 307 tests pass
- Clippy clean on modified crates