feat: add DeepSeek, Moonshot, xAI, Groq, Ollama; drop v1; migrate TUI to v2

Five OpenAI-compatible providers join the library as first-class constructors
(llm.DeepSeek, llm.Moonshot, llm.XAI, llm.Groq, llm.Ollama). Their wire-level
implementation is shared via a new v2/openaicompat package which is the
extracted guts of the old v2/openai provider; each provider supplies its own
Rules value to declare per-model constraints (e.g., DeepSeek Reasoner rejects
tools and temperature, Moonshot/xAI accept images only on *-vision* models,
Groq rejects audio input). v2/openai itself becomes a thin wrapper that sets
RestrictTemperature for o-series and gpt-5 models.

A new provider registry (v2/registry.go) exposes llm.Providers() and drives
the TUI's provider picker so adding a provider in future is a single-file
change.

The TUI at cmd/llm was migrated from v1 to v2 and moved to v2/cmd/llm. With
nothing else depending on v1, the v1 code at the repo root (all .go files,
schema/, internal/, provider/, root go.mod/go.sum) is deleted.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

v2/cmd/llm/.env.example

@@ -0,0 +1,27 @@
+# go-llm CLI environment variables
+# Copy this file to .env and fill in the keys for providers you use.
+
+# OpenAI API Key (https://platform.openai.com/api-keys)
+OPENAI_API_KEY=
+
+# Anthropic API Key (https://console.anthropic.com/settings/keys)
+ANTHROPIC_API_KEY=
+
+# Google AI API Key (https://aistudio.google.com/apikey)
+GOOGLE_API_KEY=
+
+# DeepSeek API Key (https://platform.deepseek.com)
+DEEPSEEK_API_KEY=
+
+# Moonshot / Kimi API Key (https://platform.moonshot.ai)
+MOONSHOT_API_KEY=
+
+# xAI / Grok API Key (https://x.ai/api)
+XAI_API_KEY=
+
+# Groq API Key (https://console.groq.com/keys)
+GROQ_API_KEY=
+
+# Ollama runs locally with no API key required.
+# Override the endpoint if you're not using localhost:11434.
+# OLLAMA_BASE_URL=http://localhost:11434/v1