docs: Add serial console and provisioning to cheatsheet

docs/CHEATSHEET.md

@@ -213,7 +213,7 @@ Sensors register their hostname via mDNS on boot.
 ## HMAC Command Authentication
 
 ```bash
-# Set auth secret on device
+# Set auth secret on device (requires existing auth or serial access)
 esp-ctl cmd amber-maple.local "AUTH mysecretkey123"
 
 # Set env var so all tools sign commands automatically
@@ -221,13 +221,37 @@ export ESP_CMD_SECRET="mysecretkey123"   # add to ~/.bashrc.secrets
 
 # All esp-cmd/esp-ctl/esp-fleet/esp-ota commands auto-sign when ESP_CMD_SECRET is set
 # Unsigned commands are rejected with "ERR AUTH required"
-
-# To disable auth, use FACTORY reset via serial or NVS erase
 ```
 
 Protocol: `HMAC:<32hex>:<uptime_s>:<cmd>` — first 32 hex chars of HMAC-SHA256(secret, `<uptime_s>:<cmd>`).
 Replay window: +/-5s from device uptime.
 
+### Serial Console (physical access)
+
+Connect via USB serial (921600 baud) for auth management without network auth:
+
+```bash
+# Connect to serial console
+idf.py -p /dev/ttyUSB0 monitor          # or: screen /dev/ttyUSB0 921600
+
+# Serial commands (type directly):
+AUTH                                      # Show full secret (unredacted)
+AUTH <secret>                             # Set new secret (8-64 chars)
+AUTH OFF                                  # Clear secret (disable auth)
+STATUS                                    # Basic device info
+HELP                                      # List serial commands
+```
+
+### Provisioning Tool
+
+```bash
+esp-provision                             # Auto-generate secret, set via serial
+esp-provision mysecretkey123              # Set specific secret via serial
+esp-provision --serial                    # Set via serial console (device running)
+esp-provision --generate-only             # Just print a random secret
+esp-provision -p /dev/ttyACM0             # Use different serial port
+```
+
 ## OUI Vendor Lookup
 
 ```bash