feat: support pentests with Codex subscription auth (#419)

docs/ai-providers.md

@@ -144,12 +144,55 @@ The variable is rejected in preflight where it cannot take effect: with a non-`o
 
 `npx @keygraph/shannon setup` covers this under **Custom Base URL**, which asks which API your gateway serves and configures the matching provider for you.
 
+## OpenAI Codex (ChatGPT Plus/Pro subscription)
+
+A ChatGPT Plus or Pro Codex subscription can run Shannon. Shannon reuses a login created by Pi.
+
+Before running a pentest, review the [cyber safeguards requirements](#cyber-safeguards-do-this-before-your-first-scan).
+
+1. Install Pi by following the instructions at [pi.dev](https://pi.dev).
+2. Log in with your subscription using Pi's [subscription authentication guide](https://pi.dev/docs/latest/providers#subscriptions). This creates `~/.pi/agent/auth.json` with an `openai-codex` entry.
+
+3. Select a Codex model and enable Pi authentication:
+
+   ```bash
+   export SHANNON_USE_PI_AUTH=1
+   export SHANNON_AI_MODEL=openai-codex:gpt-5.5
+   ```
+
+4. In npx mode, run `npx @keygraph/shannon start ...` from the same shell. In source-build mode, add the two variables to `.env` and run `./shannon start ...`.
+
+Supported Codex models are `gpt-5.6-sol`, `gpt-5.5`, and `gpt-5.4`.
+
+## Claude Code subscription
+
+The latest version of Shannon does not support Claude Code subscriptions. The [`shannon-v1`](https://github.com/KeygraphHQ/shannon/tree/shannon-v1) branch is the final release built on the Claude Agent SDK and supports Claude Code OAuth.
+
+Before running a pentest, review the [cyber safeguards requirements](#cyber-safeguards-do-this-before-your-first-scan).
+
+1. Generate a Claude Code OAuth token:
+
+   ```bash
+   claude setup-token
+   ```
+
+2. Run the setup flow for the final `shannon-v1` release:
+
+   ```bash
+   npx @keygraph/shannon@1.9.0 setup
+   ```
+
+3. Select **OAuth Token** and enter the token generated by Claude Code.
+4. Start the pentest with `npx @keygraph/shannon@1.9.0 start ...`.
+
+These instructions apply only to `shannon-v1`.
+
 ## Validation
 
 Checks run before a scan starts, so mistakes fail immediately rather than partway through a run:
 
 - **Provider and model ID** — validated against the Pi harness catalogue. An unknown provider or model ID fails preflight with a pointer to [pi.dev/models](https://pi.dev/models). A custom base URL exempts the model ID, since a gateway may serve its own names.
-- **Credential presence** — always validated for the selected provider.
+- **Credential presence** — validated for the selected provider, or read from Pi when `SHANNON_USE_PI_AUTH=1`.
 - **Credential validity** — one minimal request against the model the scan will use, so a rejected key, an exhausted quota, or a model the account cannot reach fails before any agent runs. Bedrock included: its bearer token and region go through the same probe.
 
 ## Migrating from the three-tier configuration