Add --list-processes to list running -r/--run sessions

Lists every session pid file found under
$XDG_STATE_HOME/slocker-lite/run/, showing pid, container name, and
status (running or exited). Status is determined the same way any
external tool could check it: a non-blocking exclusive flock() on the
file that succeeds means it's actually stale (nothing holds it), so
the session is reported as exited in that case; the lock is always
released again immediately, never left held by the check itself.

list_sessions() (pid_file.{h,cpp}) reads the real pid from each file's
own contents rather than parsing it out of the filename, which would
be ambiguous for container names that themselves contain '-'.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

CLAUDE.md

@@ -18,7 +18,13 @@ Source layout (all under `src/`):
 - `main.cpp` — CLI entry point, dependency checks, orchestration (`mount_image()`,
   `run_container()`, `cleanup_image()`, `unmount_image()`, `list_images_command()`,
   `inspect_image_command()`, `create_volume_command()`, `list_volumes_command()`,
-  `delete_volume_command()`). `inspect_image_command()` implements `-i/--inspect
+  `delete_volume_command()`, `list_processes_command()`). `list_processes_command()`
+  implements `--list-processes` (long-option only): calls `list_sessions()`
+  (`pid_file.{h,cpp}`, see below) and prints one tab-aligned `pid`, `container
+  name`, `running`/`exited` row per entry (same two-column tab-alignment scheme
+  as `list_images_command()`/`list_volumes_command()`, extended to a third
+  column), no header row, silent success on an empty list.
+  `inspect_image_command()` implements `-i/--inspect
   <image.tar>`: prints every `OciImageConfig` field (user/group, exposed ports, env,
   volumes, default command) without mounting or running the image — extend it
   whenever `OciImageConfig` gains a new field (see `oci_image.{h,cpp}` below).
@@ -188,7 +194,19 @@ Source layout (all under `src/`):
   closes the fd (releasing the flock immediately) and removes the file. Every
   failure path here (can't create the directory/file, can't lock, can't remove)
   is a `spdlog::warn`, never fatal — session tracking is best-effort and must
-  never block or fail `-r/--run` itself.
+  never block or fail `-r/--run` itself. `list_sessions()` implements
+  `--list-processes` (`main.cpp`'s `list_processes_command()`): scans the same
+  `run/` directory and reports one `SessionInfo {pid, container_name, running}`
+  per readable pid file. `pid` is read from the file's own contents, not parsed
+  from the filename (ambiguous for names that themselves contain `-`);
+  `container_name` is then recovered by stripping that exact `-<pid>` suffix
+  back off the filename. `running` reuses the same liveness check any external
+  tool would do — a non-blocking exclusive `flock()` that succeeds means the
+  file is actually stale, so `running` is false in that case; the lock is always
+  released again immediately either way, never left held by the check itself. A
+  file that can't be opened or doesn't parse as a pid (e.g. removed mid-scan) is
+  silently skipped, not reported as an error — scanning a live directory is
+  inherently racy.
 - `config_file.{h,cpp}` — `load_config_file()` reads and parses (via libyaml's
   document API, `<yaml.h>`) the `global` and `volumes` sections of the local YAML
   config file located by `config_file_path()` (`$XDG_CONFIG_HOME/slocker-lite/config.yaml`,
@@ -267,7 +285,8 @@ Build directory is `buildDir/` (already configured).
   full flag list: `-m/--mount`, `-r/--run`, `-u/--umount`, `-c/--cleanup`,
   `-l/--list-images`, `-i/--inspect`, `-n/--no-nsenter`, `--user`, `--group`,
   `--hostname`, `-v/--volume`, `--list-volumes`, `--delete-volume`,
-  `--delete-volume-full`, `-t/--test`, `--log-level`, `-h/--help`, `-V/--version`)
+  `--delete-volume-full`, `--list-processes`, `-t/--test`, `--log-level`,
+  `-h/--help`, `-V/--version`)
 - Run tests: `meson test -C buildDir`
 
 ## Code style