docs: document --debug flag and extraArgs module option

Add documentation for:
- --debug flag in Listener Flags table
- --heartbeat-interval flag (was missing)
- extraArgs NixOS module option
- New Troubleshooting section with debug logging examples
  and guidance for diagnosing metrics issues

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -63,6 +63,8 @@ homelab-deploy listener \
 | `--discover-subject` | No | Discovery subject (default: `deploy.discover`) |
 | `--metrics-enabled` | No | Enable Prometheus metrics endpoint |
 | `--metrics-addr` | No | Metrics HTTP server address (default: `:9972`) |
+| `--heartbeat-interval` | No | Status update interval in seconds during deployment (default: 15) |
+| `--debug` | No | Enable debug logging for troubleshooting |
 
 #### Subject Templates
 
@@ -214,6 +216,7 @@ Add the module to your NixOS configuration:
 | `metrics.enable` | bool | `false` | Enable Prometheus metrics endpoint |
 | `metrics.address` | string | `":9972"` | Metrics HTTP server address |
 | `metrics.openFirewall` | bool | `false` | Open firewall for metrics port |
+| `extraArgs` | list of string | `[]` | Extra command line arguments (e.g., `["--debug"]`) |
 
 Default `deploySubjects`:
 ```nix
@@ -298,6 +301,57 @@ histogram_quantile(0.95, rate(homelab_deploy_deployment_duration_seconds_bucket[
 sum(homelab_deploy_deployment_in_progress)
 ```
 
+## Troubleshooting
+
+### Debug Logging
+
+Enable debug logging to diagnose issues with deployments or metrics:
+
+**CLI:**
+```bash
+homelab-deploy listener --debug \
+  --hostname myhost \
+  --tier prod \
+  --nats-url nats://nats.example.com:4222 \
+  --nkey-file /run/secrets/listener.nkey \
+  --flake-url git+https://git.example.com/user/nixos-configs.git \
+  --metrics-enabled
+```
+
+**NixOS module:**
+```nix
+services.homelab-deploy.listener = {
+  enable = true;
+  tier = "prod";
+  natsUrl = "nats://nats.example.com:4222";
+  nkeyFile = "/run/secrets/homelab-deploy-nkey";
+  flakeUrl = "git+https://git.example.com/user/nixos-configs.git";
+  metrics.enable = true;
+  extraArgs = [ "--debug" ];
+};
+```
+
+With debug logging enabled, the listener outputs detailed information about metrics recording:
+
+```json
+{"level":"DEBUG","msg":"recording deployment start metric","metrics_enabled":true}
+{"level":"DEBUG","msg":"recording deployment end metric (success)","action":"switch","success":true,"duration_seconds":120.5}
+```
+
+### Metrics Showing Zero
+
+If deployment metrics remain at zero after deployments:
+
+1. **Check metrics are enabled**: Verify `--metrics-enabled` is set and the metrics endpoint is accessible at `/metrics`
+
+2. **Enable debug logging**: Use `--debug` to confirm metrics recording is being called
+
+3. **Check deployment status**: Metrics are only recorded for deployments that complete (success or failure). Rejected requests (e.g., already running) increment the counter with `status="rejected"` but don't record duration
+
+4. **Check after restart**: After a successful `switch` deployment, the listener restarts. Metrics reset to zero in the new instance. The listener waits up to 60 seconds for a Prometheus scrape before restarting to capture the final metrics
+
+5. **Verify Prometheus scrape timing**: Ensure Prometheus scrapes frequently enough to capture metrics before the listener restarts
+
 ## Message Protocol
 
 ### Deploy Request