docs: add .env.example configuration template

Author: JoshuaOliphant

.env.example

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+# ============================================================
+# Herald Configuration
+# Copy this file to .env and fill in your values
+# ============================================================
+
+# --- Required ---
+
+# Telegram bot token from @BotFather
+TELEGRAM_BOT_TOKEN=your_bot_token_here
+
+# Comma-separated list of allowed Telegram user IDs
+# Get yours from @userinfobot on Telegram
+ALLOWED_TELEGRAM_USER_IDS=123456789
+
+# --- Paths ---
+
+# Path to your second brain / knowledge base directory
+# SECOND_BRAIN_PATH=/path/to/your/second-brain
+
+# Herald memory directory (relative to SECOND_BRAIN_PATH)
+# MEMORY_PATH=areas/herald
+
+# Override chat history location (relative to SECOND_BRAIN_PATH)
+# CHAT_HISTORY_PATH_OVERRIDE=custom/chat-history
+
+# --- Server ---
+
+# HOST=0.0.0.0
+# PORT=8080
+# WEBHOOK_PATH=/webhook
+
+# Max execution time per message (seconds)
+# COMMAND_TIMEOUT=300
+
+# --- Claude ---
+
+# Override Claude model (e.g., claude-opus-4-6, claude-sonnet-4-5-20250929)
+# CLAUDE_MODEL=
+
+# Enable experimental agent teams (multi-agent for complex tasks)
+# AGENT_TEAMS=false
+
+# --- Heartbeat (Proactive Check-ins) ---
+
+# Enable periodic heartbeat checks
+# HEARTBEAT_ENABLED=false
+
+# Check interval (e.g., 15m, 1h, 2h30m)
+# HEARTBEAT_EVERY=30m
+
+# Custom prompt for heartbeat checks (inline)
+# HEARTBEAT_PROMPT=Check for overdue tasks and unprocessed inbox items
+
+# Load heartbeat prompt from file (overrides HEARTBEAT_PROMPT)
+# HEARTBEAT_PROMPT_FILE=/path/to/heartbeat-prompt.md
+
+# Where to deliver heartbeat alerts: "last" (most recent chat) or a chat ID
+# HEARTBEAT_TARGET=last
+
+# Only run heartbeats during these hours (24h format, in HEARTBEAT_TIMEZONE)
+# HEARTBEAT_ACTIVE_HOURS=08:00-22:00
+
+# Timezone for active hours (e.g., America/New_York, Europe/London)
+# HEARTBEAT_TIMEZONE=UTC
+
+# Suppress "all clear" responses shorter than this
+# HEARTBEAT_ACK_MAX_CHARS=300
+
+# Model override for heartbeats (e.g., sonnet for cheaper checks)
+# HEARTBEAT_MODEL=
+
+# Path to HEARTBEAT.md file injected into heartbeat context
+# HEARTBEAT_FILE=