Harden Python heredocs against shell injection

## Description

Several AWS functions use unquoted heredocs (`<< EOF`) to pass Python code to `python3`. This allows shell variable expansion (`${variable}`) inside the Python code, which means user-controlled values (instance names, regions) are interpolated directly into Python source.

While input validation has been added (via `validate_name()` and `validate_region()`), the underlying pattern is still fragile. A defense-in-depth approach would pass values via environment variables instead.

## Current pattern

```bash
python3 << EOF
region = '${region}'
instance_id = '${instance_id}'
...
EOF
```

## Recommended pattern

```bash
REGION="$region" INSTANCE_ID="$instance_id" python3 << 'EOF'
import os
region = os.environ['REGION']
instance_id = os.environ['INSTANCE_ID']
...
EOF
```

## Affected functions

- `aws_create` — instance creation Python block
- `aws_destroy` — instance termination Python block
- `aws_info` — instance info Python block
- `aws_stop` — instance stop Python block
- `aws_start` — instance start Python block
- `aws_update` — instance update Python block
- `aws_update_ip` — IP update Python block
- `aws_sync` — instance discovery Python block
- `select_ssm_instance_profile` — SSM profile selection

## Priority

Low — mitigated by input validation added in recent commit.

Locate the Python heredoc blocks in the named functions: aws_create, aws_destroy, aws_info, aws_stop, aws_start, aws_update, aws_update_ip, aws_sync, and select_ssm_instance_profile. Compare each block with the recommended quoted-heredoc and environment-variable pattern, then verify that all listed user-controlled values are passed without shell expansion. Done means every affected block uses the hardened pattern while preserving its existing behavior.

Written by the indexing model from the issue text.