.env file is sourced into kiri-cli shell on load

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 26.1.0 (25B78)

### Expected behaviour

Environment variable in .env files shouldn't be loaded by the agent's shell uses to run commands.

The agent's shell should instead inherit the user's shell environment only (what's in .zshrc/.zprofile), not auto-load .env files. If .env loading is intentional, it should be opt-in or documented.

### Actual behaviour

Environment variable are sourced into the terminal kiro-cli uses to run commands.
This gives the agent access to secret is doesn't need and should have access to. Also this can also effects the result of some automated test, for example if the log level is set in the .env file. This causes confusion with the agent getting different test results to the user.

### Steps to reproduce

1. Create an .env file
2. Add an environment variable to the file. eg: `SECRET_NOT_MEANT_FOR_AGENTS="Oh No, I leaked!"`
3. From the the directory the `.env` file was created, open `kiro-cli`
4. Inside the kiro-cli `echo` one of the environment variables from the `.env` file. eg `!echo $SECRET_NOT_MEANT_FOR_AGENTS`
5. The value of the environment variable is displayed on the screen

### Environment

```yaml

[q-details]
version = "2.4.0"
hash = "da457ad7e4bc0693d1f094f7571fb87091a0e406"
date = "2026-05-20T19:18:35.988733Z (18h ago)"
variant = "full"

[system-info]
os = "macOS 26.1.0 (25B78)"
chip = "Apple M4 Pro"
total-cores = 14
memory = "48.00 GB"

[environment]
os = "Mac"
shell-path = "/bin/zsh"
shell-version = "5.9"
terminal = "iTerm 2"
install-method = "unknown"

[env-vars]
QTERM_SESSION_ID = "d3ee10ca255b4e9eabaf7148839cd630"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "2.4.0"
SHELL = "/bin/zsh"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.googlecode.iterm2"
```

Start by reproducing the behavior from a directory containing a .env file, using the kiro-cli shell and the documented echo command. Trace how the agent's shell environment is initialized; done means .env variables are not inherited while variables from the user's zsh environment remain available, with the behavior either made opt-in or documented.

Written by the indexing model from the issue text.