feat: openclaw-style secrets (env.vars + \) and per-task model routing

- Replace python-dotenv with config.json env.vars block + \ substitution
- Add models section for per-task model routing (heartbeat, subagent, default)
- Heartbeat/subagent tasks can use different models/providers than main chat
- Remove python-dotenv from dependencies
- Update all docs to reflect new config approach
- Reorganize docs into project/ and research/ subdirectories

.env.example

@@ -1,23 +1,30 @@
 # =============================================================================
-# Aetheel Secrets (.env)
+# Aetheel .env (LEGACY — optional fallback)
 # =============================================================================
-# This file holds SECRETS ONLY (tokens, passwords, API keys).
-# All other configuration lives in ~/.aetheel/config.json.
+# Secrets now live in ~/.aetheel/config.json under the "env.vars" block.
+# Process environment variables still override everything.
 #
-# Copy this file to .env and fill in your values.
+# Example config.json:
+#
+#   {
+#     "env": {
+#       "vars": {
+#         "SLACK_BOT_TOKEN": "xoxb-your-bot-token",
+#         "DISCORD_BOT_TOKEN": "your-discord-token"
+#       }
+#     },
+#     "discord": {
+#       "enabled": true,
+#       "bot_token": "${DISCORD_BOT_TOKEN}"
+#     }
+#   }
+#
+# If you still prefer a .env file, set these and they will be picked up
+# from the process environment as a final override.
 
-# --- Slack Tokens ------------------------------------------------------------
-SLACK_BOT_TOKEN=xoxb-your-bot-token-here
-SLACK_APP_TOKEN=xapp-your-app-token-here
-
-# --- Telegram Bot Token (optional) -------------------------------------------
+# SLACK_BOT_TOKEN=xoxb-your-bot-token-here
+# SLACK_APP_TOKEN=xapp-your-app-token-here
 # TELEGRAM_BOT_TOKEN=your-telegram-bot-token-here
-
-# --- Discord Bot Token (optional) --------------------------------------------
 # DISCORD_BOT_TOKEN=your-discord-bot-token-here
-
-# --- OpenCode Server Password (optional, SDK mode only) ----------------------
 # OPENCODE_SERVER_PASSWORD=your-server-password
-
-# --- Anthropic API Key (optional, for Claude Code runtime) -------------------
 # ANTHROPIC_API_KEY=sk-ant-...