docs: Add ColdStarter system documentation (#26)

Verified against source code, cut by 50%+ per feedback

Author: druid-infra

docs/cli/coldstarter.md

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+---
+sidebar_position: 2
+title: ColdStarter
+description: Wake-on-demand system for cost savings
+---
+
+# ColdStarter
+
+ColdStarter automatically wakes your server when players connect, then puts it to sleep when idle. You only pay for active time.
+
+## How It Works
+
+1. **Idle**: No players → server sleeps (minimal cost)
+2. **Connect**: Player tries to join → ColdStarter intercepts
+3. **Wake**: Server starts in background (10-30 seconds)
+4. **Play**: Player connects to running server
+
+Your server still shows as "online" in server browsers while sleeping.
+
+## Cost Savings
+
+Typical hobby server:
+- **Without ColdStarter**: 720 hours/month (24/7)
+- **With ColdStarter**: 100 hours/month (actual playtime)
+
+**Result**: ~85% cost reduction
+
+## Configuration
+
+Configured per-scroll via `scroll.yaml`:
+
+```yaml
+ports:
+  - name: game
+    port: 25565
+    protocol: tcp
+    sleep_handler: packet_handler/minecraft.lua
+    start_delay: 10
+    check_activity: true
+```
+
+### Port Options
+
+- `sleep_handler` - Path to Lua packet handler (required for ColdStarter)
+- `start_delay` - Seconds to wait before port is ready
+- `check_activity` - Enable idle detection
+- `finish_after_command` - Wait for command to finish before opening port
+
+## Packet Handlers
+
+Lua handlers respond to game protocols while server is asleep.
+
+**Example:** [minecraft.lua](https://github.com/highcard-dev/scrolls/blob/main/scrolls/minecraft/papermc/1.21.7/packet_handler/minecraft.lua)
+
+### Lua API
+
+```lua
+-- Send data to client
+sendData(string)
+
+-- Trigger server wake
+finish()
+
+-- Get snapshot/queue status
+get_queue()
+get_snapshot_percentage()
+get_finish_sec()
+```
+
+## Supported Games
+
+All 95 published scrolls support ColdStarter:
+- Minecraft (all variants)
+- Rust (Vanilla, Oxide)
+- Hytale
+- 10 LGSM games (Palworld, ARK, CS2, etc.)
+
+## Custom Handlers
+
+Create `packet_handler/game.lua` in your scroll:
+
+```lua
+function handle(ctx, data)
+    -- Parse packet
+    if isStatusRequest(data) then
+        sendData("Server starting...")
+    end
+    
+    -- Wake on connect
+    if isConnectionAttempt(data) then
+        finish()
+    end
+end
+```