Add structured daemon lifecycle logs

VM start, image build, and network/setup failures were hard to diagnose because bangerd emitted almost no lifecycle logs and the Firecracker SDK logger was discarded. This adds a daemon-wide JSON logger with configurable log level so failures leave breadcrumbs instead of only side effects.

Log the main daemon and VM lifecycle stages, preserve raw Firecracker and image-build helper output in dedicated files, and include those log paths in daemon status and returned errors. Bridge SDK logrus output into the daemon logger at debug level so low-level Firecracker diagnostics are available without making normal info logs unreadable.

Validation: go test ./... and make build. Left unrelated worktree changes out of this commit, including internal/api/types.go, the deleted shell scripts, and my-rootfs.ext4.

README.md

@@ -106,6 +106,9 @@ banger daemon socket
 banger daemon stop
 ```
 
+`banger daemon status` prints the daemon PID, socket path, and `bangerd.log`
+location.
+
 State lives under XDG directories:
 - config: `~/.config/banger`
 - state: `~/.local/state/banger`
@@ -121,6 +124,7 @@ repo-built `./banger`. You can override either with `runtime_dir` in
 `BANGER_MAPDNS_DATA_FILE`.
 
 Useful config keys:
+- `log_level`
 - `runtime_dir`
 - `firecracker_bin`
 - `mapdns_bin`
@@ -135,6 +139,15 @@ Useful config keys:
 - `default_modules_dir`
 - `default_packages_file`
 
+## Logs
+- daemon lifecycle logs: `~/.local/state/banger/bangerd.log`
+- raw Firecracker output per VM: `~/.local/state/banger/vms/<vm-id>/firecracker.log`
+- raw image-build helper output: `~/.local/state/banger/image-build/*.log`
+
+`bangerd.log` is structured JSON. Set `log_level` in
+`~/.config/banger/config.toml` or `BANGER_LOG_LEVEL` to one of `debug`,
+`info`, `warn`, or `error`.
+
 ## Images
 List images:
 ```bash