feat(preflight): add exploit-readiness probe and --validate-auth mode, and refresh suggested models (#476)

* feat(preflight): gate scans on an exploit-workload readiness probe

* feat: add --validate-auth to run authentication validation only

* feat: refuse reusing an auth-validation workspace for a scan

* chore: refresh suggested model IDs (Grok 4.7, OpenAI gpt-6-sol, Claude 5)

* fix(preflight): make the exploit-readiness probe trip the cyber safeguard reliably

* chore(preflight): update the exploit-readiness probe prompt

* feat: add --validate-model to run the preflight model checks only

* feat(cli): name cyber-access and app-login steps in the start loader

* feat(cli): refine start loader — skip app-login step when following, annotate preflight label

* feat(status): show Preflight and Cyber access verification rows for gated providers

* chore(preflight): suggest a fallback model in cyber-access remediation hints

* chore(preflight): drop env-var syntax from cyber-access fallback hints

* fix(preflight): separate finding heading from Target line in readiness probe

* refactor(preflight): rename exploit-readiness probe to cyber access verification

* fix(preflight): re-join finding heading with Target line in readiness probe

Reverts the heading/Target split from 22d84f2, gluing each finding's
heading back onto its Target line in the cyber-access probe's user
content.

* feat(validation): show a Checks summary in the validation log

* fix(cli): say a validation run failed, not that it could not start

* docs(ai-providers): replace broken Pi subscription link with /login steps

* feat(preflight): gate the openai-codex subscription on cyber access

docs/configuration.md

@@ -179,3 +179,14 @@ login_flow:
   - "If prompted for 2FA, type $totp in <exact code field label or placeholder>"
   - "Click <exact button text>"
 ```
+
+### Validating Authentication Only
+
+To confirm your login flow works before committing to a full scan, add `--validate-auth` to `start`:
+
+```bash
+npx @keygraph/shannon start -u https://your-app.com -r /path/to/repo -c config.yaml --validate-auth
+```
+
+The run performs preflight and the single real login, then stops. No pentest, reconciliation, or report
+is produced. It requires an `authentication` block in the config.