Add -e/--exec to join a running -r/--run session

Validates the given pid against the same tracked-session liveness
check --list-processes/--clean-processes already use, then joins its
namespaces via nsenter and runs a command there in the foreground.

Two things discovered only by testing against a live session, not
assumed up front:

- The tracked pid is bwrap's own outer process. It sets up the
  mount/user namespaces itself, then clone()s the actual sandboxed
  command into fresh pid/uts/ipc/cgroup namespaces -- clone()'s
  namespace flags only ever affect the new child, never the caller,
  so the outer process itself never enters those namespaces at all.
  exec_in_session() resolves that real child via
  /proc/<pid>/task/<pid>/children and joins its namespaces instead,
  falling back to the outer pid if that can't be read.

- Rather than nsenter -a (which would hit a known "Invalid argument"
  failure re-entering an identical namespace -- this project already
  worked around exactly that once, for the containers-storage mount
  path), each namespace type is only joined if
  /proc/<pid>/ns/<type> actually differs from this process's own.
  nsenter also needs --preserve-credentials, or it tries to
  setuid/setgid/setgroups to the target's identity, which fails
  outright against the setgroups-denied unprivileged user namespace
  bwrap creates whenever -r/--run isn't root.

Verified end-to-end: joined shell gets the container's own hostname,
process tree (ps shows only container processes), and root
filesystem; untracked/stale pids error out cleanly without touching
nsenter; Ctrl-C during the joined command doesn't disturb the
original session; no leftover mounts after either exits.

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

README.md

@@ -58,6 +58,7 @@ slocker-lite -u|--umount <layer-id>
 slocker-lite -c|--cleanup <layer-id>
 slocker-lite -l|--list-images <directory>
 slocker-lite -i|--inspect <image.tar>
+slocker-lite -e|--exec <pid> [-- <command> [args...]]
 slocker-lite -v|--volume <name> <directory>
 slocker-lite --list-volumes
 slocker-lite --delete-volume <name>
@@ -81,6 +82,7 @@ slocker-lite -V|--version
 | `--hostname <name>` | With `--run`, set the sandbox's hostname. Only takes effect if the running kernel supports `--unshare-uts`; ignored with a warning otherwise. |
 | `-l, --list-images <dir>` | List OCI Image Layout tars (`*.tar`, `*.tar.*`) found directly in `<dir>`, with their `name:tag`. |
 | `-i, --inspect <image.tar>` | Print an image's declared user, exposed ports, env, volumes, and default command, without mounting or running it. |
+| `-e, --exec <pid>` | Join an already-running `--run` session (`<pid>` must be one `--list-processes` shows as `running`) and run a command inside its container. Pass `-- <command> [args...]` to specify it. |
 | `-v, --volume <name> <dir>` | Create a named volume mapped to a host directory (created if missing), recorded in the config file's `volumes` section. Fails if the name or directory is already used by an existing volume. Volume names can't contain `/`. With `--run`, instead mounts a volume into the sandbox (repeatable): `<name>` is an existing named volume, or, if it contains `/`, a host directory path (created if missing); `<dir>` is the absolute path inside the container to mount it at. If the host directory is empty and the image already has content there, that content is copied in first, preserving numeric ownership/permissions/links and, where the host filesystem supports them, extended attributes/ACLs (skipped with a warning otherwise). |
 | `--list-volumes` | List all named volumes (see `-v/--volume`) with their host directory. |
 | `--delete-volume <name>` | Remove a named volume from the config. The host directory is left untouched. |
@@ -116,6 +118,12 @@ sudo ./buildDir/slocker-lite -r myimage.tar --user git
 # Inspect an image's declared config without mounting or running it
 ./buildDir/slocker-lite -i myimage.tar
 
+# Start a busybox container in the background, then get a shell inside it from
+# another terminal (find its pid with --list-processes)
+./buildDir/slocker-lite -r busybox.tar &
+./buildDir/slocker-lite --list-processes
+./buildDir/slocker-lite -e 12345 -- /bin/sh
+
 # Create a named volume backed by a host directory
 ./buildDir/slocker-lite -v mydata ~/slocker-volumes/mydata
 
@@ -187,6 +195,14 @@ when its own session ends, but `--clean-processes` removes any stale ones left
 behind (e.g. after a crash) using that same check, atomically per file, so it
 never removes one that's still genuinely running.
 
+`-e/--exec <pid>` joins a running session's namespaces with `nsenter` and runs a
+command there. Because `bwrap` itself sets up the sandbox's mount/user namespaces
+and then hands the actual sandboxed command off to a child process in fresh
+pid/uts/ipc/cgroup namespaces, `-e` resolves that real child first (via `/proc`)
+rather than joining the outer `bwrap` process's own namespaces, so the joined
+command sees the container's process tree and hostname too, not just its
+filesystem.
+
 See `CLAUDE.md` for the full architecture writeup (file-by-file breakdown, the
 reasoning behind each of the above, and known gaps).