Move API credentials to .env and set gpt-5.6-luna model

- Load OPENAI_API_KEY and OPENAI_BASE_URL from gitignored .env (repo-root, CWD-independent)
- Fix command explanation to use the configured model instead of hardcoded gpt-4o-mini
- Update default model and fallbacks to gpt-5.6-luna
- Document .env-based configuration in READMEs

README.md

@@ -19,7 +19,7 @@ Arguments:
   --theme <theme>    : Set syntax highlighting theme (e.g., monokai, github-dark)
 
 Current configuration per edison.yaml:
-* Model        : gpt-4o-mini
+* Model        : gpt-5.6-luna
 * Temperature  : 0
 * Max. Tokens  : 500
 * Safety       : on
@@ -64,13 +64,27 @@ cd command-assistant
 .\install_edison.bat
 ```
 
-## OpenAI API Key Configuration
+## API Configuration
 
-Configure your OpenAI API key using one of these methods:
+Edison needs an API key (and, when using a proxy, a base URL) to talk to your model backend. The recommended way is a `.env` file in the repository root, which is gitignored and therefore never checked in:
 
-- Environment variable: `export OPENAI_API_KEY=<yourkey>` (Linux/macOS) or `$env:OPENAI_API_KEY="<yourkey>"` (Windows)
-- Create a file at `~/.openai.apikey` with just the key
-- Add the key to the `edison.yaml` configuration file
+```bash
+# .env (do not commit this file)
+OPENAI_API_KEY="your-api-key"
+OPENAI_BASE_URL="https://api.openai.com/v1"
+```
+
+Copy `.env.example` to get started:
+
+```bash
+cp .env.example .env
+```
+
+You can also provide the key using any of these methods (in order of precedence):
+
+- `OPENAI_API_KEY` / `OPENAI_BASE_URL` environment variables (or in `.env`)
+- A file at `~/.openai.apikey` containing just the key
+- The `openai_api_key` / `base_url` entries in `edison.yaml`
 
 ## Usage Examples