docs: update .env.example to match pkg/config variable names

Updated `.env.example` with the correct `PICOCLAW_` prefixes for environment variables.
Mapped keys directly to their respective configuration structs. Added missing configurations like DeepSeek, Volcengine, Agent Defaults (Model and Workspace), and Tavily API key to align with recent configuration updates.

Co-authored-by: hobbyistlabs-coder <267281733+hobbyistlabs-coder@users.noreply.github.com>

.env.example

@@ -1,17 +1,28 @@
+# ── Core ──────────────────────────────────
+# PICOCLAW_HOME=~/.picoclaw
+# PICOCLAW_CONFIG=~/.picoclaw/config.json
+
+# ── Agent Defaults ────────────────────────
+# PICOCLAW_AGENTS_DEFAULTS_MODEL=openai/gpt-4o
+# PICOCLAW_AGENTS_DEFAULTS_WORKSPACE=~/.picoclaw/workspace
+
 # ── LLM Provider ──────────────────────────
 # Uncomment and set the API key for your provider
-# OPENROUTER_API_KEY=sk-or-v1-xxx
-# ZHIPU_API_KEY=xxx
-# ANTHROPIC_API_KEY=sk-ant-xxx
-# OPENAI_API_KEY=sk-xxx
-# GEMINI_API_KEY=xxx
-# CLAUDE_CODE_OAUTH=xxx
+# PICOCLAW_PROVIDERS_OPENROUTER_API_KEY=sk-or-v1-xxx
+# PICOCLAW_PROVIDERS_ZHIPU_API_KEY=xxx
+# PICOCLAW_PROVIDERS_ANTHROPIC_API_KEY=sk-ant-xxx
+# PICOCLAW_PROVIDERS_OPENAI_API_KEY=sk-xxx
+# PICOCLAW_PROVIDERS_GEMINI_API_KEY=xxx
+# PICOCLAW_PROVIDERS_DEEPSEEK_API_KEY=sk-xxx
+# PICOCLAW_PROVIDERS_VOLCENGINE_API_KEY=sk-xxx
+
 # ── Chat Channel ──────────────────────────
-# TELEGRAM_BOT_TOKEN=123456:ABC...
-# DISCORD_BOT_TOKEN=xxx
+# PICOCLAW_CHANNELS_TELEGRAM_TOKEN=123456:ABC...
+# PICOCLAW_CHANNELS_DISCORD_TOKEN=xxx
 
 # ── Web Search (optional) ────────────────
-# BRAVE_SEARCH_API_KEY=BSA...
+# PICOCLAW_TOOLS_WEB_BRAVE_API_KEY=BSA...
+# PICOCLAW_TOOLS_WEB_TAVILY_API_KEY=tvly-...
 
 # ── Timezone ──────────────────────────────
 TZ=Asia/Shanghai