feat: dynamic provider config — pos config ai auto-discovers provider keys

- lib/ai-providers/*.sh declare # PROVIDER_CONFIG: headers
- lib/config-ui.sh: _cfg_provider_keys() scans providers at runtime
- bin/pos-ai: POS_CONFIG uses *providers marker (no hardcoded keys)
- Adding a new provider auto-populates config UI — no main tool edits needed

lib/ai-providers/gemini.sh

@@ -3,6 +3,10 @@
 # Provider-specific: API call, auth, response parsing, models list
 # Part of the R8 provider-agnostic architecture (lib/ai-providers/).
 
+# Provider-specific config variables (auto-discovered by pos config ai):
+# PROVIDER_CONFIG: AI_GEMINI_API_KEY=secret:Gemini API key from aistudio.google.com
+# PROVIDER_CONFIG: AI_GEMINI_MODEL=:Gemini model id (default: gemini-2.5-flash)
+
 provider_name() { printf 'Google Gemini'; }
 provider_default_model() { printf 'gemini-2.5-flash'; }
 

lib/ai-providers/openrouter.sh

@@ -3,6 +3,10 @@
 # Provider-specific: API call, auth, response parsing, models list
 # Part of the R8 provider-agnostic architecture (lib/ai-providers/).
 
+# Provider-specific config variables (auto-discovered by pos config ai):
+# PROVIDER_CONFIG: OPENROUTER_API_KEY=secret:OpenRouter API key from openrouter.ai
+# PROVIDER_CONFIG: OPENROUTER_MODEL=:OpenRouter model id (default: openrouter/auto)
+
 provider_name() { printf 'OpenRouter'; }
 provider_default_model() { printf 'openrouter/auto'; }