refactor: ai — merge gemini/openrouter into unified plugin architecture

- bin/pos-ai: single provider-agnostic tool (ask/chat/sessions/capture/models/providers)
- lib/ai-providers/gemini.sh: Gemini adapter (59 ln)
- lib/ai-providers/openrouter.sh: OpenRouter adapter (59 ln)
- bin/pos-ai-gemini/openrouter: thin forwarders for backward compat
- Provider adapter interface: provider_name/default_model/generate/models_list
- Unified session format (OpenAI messages), auto-migrate old gemini contents
- Config: AI_PROVIDER/AI_API_KEY/AI_MODEL/AI_SYSTEM_PROMPT in ai.env
- Config fallback: AI_API_KEY → provider-specific env var → error
- Default system prompt configurable via AI_SYSTEM_PROMPT
- New subcommand: pos ai providers (lists providers + config status)
- Shell hook (pos-ai-hook.sh) for auto-capture

config/ai.env

@@ -1,8 +1,15 @@
-# ~/.config/linux_post_install/ai.env — Google Gemini config for 'pos ai gemini'
+# ~/.config/linux_post_install/ai.env — AI provider config for 'pos ai'
 # Copied to ~/.config/linux_post_install/ai.env on install (no clobber).
 # Edit with: pos config ai
 #
 # Syntax:
-#   AI_GEMINI_API_KEY=<key>   # required — API key from aistudio.google.com
-#                             # (never commit this to the repo)
-#   AI_GEMINI_MODEL=<model>   # optional — model id, default gemini-2.5-flash
+#   AI_PROVIDER=gemini            # provider: gemini|openrouter (default gemini)
+#   AI_API_KEY=<key>              # API key for the active provider (secret)
+#   AI_MODEL=<model>              # model id (default depends on provider)
+#   AI_SYSTEM_PROMPT=<prompt>     # custom system prompt (overrides built-in; empty to reset)
+#
+# Legacy provider-specific keys (still supported as fallback):
+#   AI_GEMINI_API_KEY=<key>       # Gemini API key (used when AI_API_KEY is empty)
+#   AI_GEMINI_MODEL=<model>       # Gemini model id (used when AI_MODEL is empty)
+#   OPENROUTER_API_KEY=<key>      # OpenRouter API key (used when AI_API_KEY is empty)
+#   OPENROUTER_MODEL=<model>      # OpenRouter model id (used when AI_MODEL is empty)