Make --kill long-option-only, no -k short form

Per the user's request: --kill is destructive enough (stops every process
a session started) that typo-prone brevity isn't worth it -- dropping the
short option makes it harder to invoke by mistake.

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

CLAUDE.md

@@ -38,7 +38,7 @@ Source layout (all under `src/`):
   `Mode::exec` dispatch arm) `-x/--exec <pid>`'s own pid parsing/validation
   (`ParsedArgs::exec_pid`, a positive integer or a hard error) and its
   trailing-command requirement (`ParsedArgs::command`, required non-empty).
-  `-k/--kill <pid>` (`ParsedArgs::kill_pid`) shares that same positive-integer
+  `--kill <pid>` (`ParsedArgs::kill_pid`) shares that same positive-integer
   parsing/validation via a small extracted `parse_pid_arg()` helper (`.cpp`-local)
   rather than duplicating the `strtol` dance a second time — unlike `-x/--exec`,
   it takes no trailing command, so it's simply not added to the leftover-args
@@ -264,7 +264,7 @@ Source layout (all under `src/`):
   same instant everything else that needs it does, rather than needing its own
   separate pid-discovery mechanism. That same `on_start` lambda also calls
   `create_session_cgroup()` (`session_cgroup.h`, see below), right alongside
-  `create_session_lock()`, so `-k/--kill` can later find every process the
+  `create_session_lock()`, so `--kill` can later find every process the
   session ever starts via its dedicated cgroup; `remove_session_cgroup()` is
   called from the same post-`run_process_foreground()` spot
   `release_session_lock()` already is.
@@ -383,7 +383,7 @@ Source layout (all under `src/`):
   a check and a later removal. Only files it actually removes are reported back
   (as `SessionInfo`s with `running=false`); still-locked (running) files are
   left untouched and not reported.
-- `session_cgroup.{h,cpp}` — gives `-k/--kill` (`kill_session.{h,cpp}`, see
+- `session_cgroup.{h,cpp}` — gives `--kill` (`kill_session.{h,cpp}`, see
   below) a reliable way to find every process a session ever started, however
   deeply forked/daemonized/reparented, by putting it in a dedicated cgroup v2
   group from the moment it starts. `cgroup_v2_available()` checks for
@@ -429,7 +429,7 @@ Source layout (all under `src/`):
   shared by `exec_session.{h,cpp}` and `kill_session.{h,cpp}` (see both
   below); pulled into their own file (rather than staying private to
   `exec_session.cpp`, where `resolve_namespace_pid()` originally lived) once
-  `-k/--kill` needed the exact same "find the real sandboxed child" logic, to
+  `--kill` needed the exact same "find the real sandboxed child" logic, to
   avoid a second, drifting copy. `resolve_namespace_pid()` is unchanged from
   its original `exec_session.cpp` form (see that entry for the full
   reasoning: bwrap's own outer/tracked pid never actually enters the
@@ -517,7 +517,7 @@ Source layout (all under `src/`):
   itself never actually enters those namespaces — comparing
   `/proc/<outer_pid>/ns/{pid,uts,ipc,cgroup}` against this process's own showed
   them identical, while only `mnt`/`user` differed. `resolve_namespace_pid()`
-  (`sandbox_process.{h,cpp}` — moved out of this file once `-k/--kill`
+  (`sandbox_process.{h,cpp}` — moved out of this file once `--kill`
   needed the exact same logic, see that entry) finds that real inner process
   so this can join *its* namespaces instead. For each of `{mnt→--mount, uts→--uts, ipc→--ipc, pid→--pid,
   cgroup→--cgroup, user→--user}` (`net` deliberately excluded — this project
@@ -584,7 +584,7 @@ Source layout (all under `src/`):
   real `250:250` via the fetched `/etc/passwd` and then fails clearly (helper not
   bind-mounted, since the session itself had no declared user) rather than
   silently running as the wrong identity.
-- `kill_session.{h,cpp}` — implements `-k/--kill <pid>`, stopping a tracked,
+- `kill_session.{h,cpp}` — implements `--kill <pid>`, stopping a tracked,
   running `-r/--run` session and everything it started. `kill_session()`
   validates `pid` the same way `exec_in_session()` does (via `list_sessions()`,
   `pid_file.h`). **Real bug reported by the user against their own actual
@@ -642,7 +642,7 @@ Source layout (all under `src/`):
      pid namespaces, unlike the user's real target device): a daemonized
      busybox session running `sh -c 'sleep 300 & exec sleep 300'` (mirroring
      the daemonize-then-exec shape of the original bug) was fully cleaned up
-     by `-k/--kill`, including the backgrounded child, with no leftover
+     by `--kill`, including the backgrounded child, with no leftover
      processes, mounts, or layers; a second run using `sh -c 'trap "" TERM;
      sleep 300'` (ignoring `SIGTERM` entirely) confirmed the forced-`SIGKILL`
      escalation path too, taking the full 10s grace period before the pid
@@ -778,7 +778,7 @@ Build directory is `buildDir/` (already configured).
   (see `priv_drop_helper.cpp` in "Project state")
 - Run the executable: `./buildDir/slocker-lite -m <image.tar>` (see `--help` for the
   full flag list: `-m/--mount`, `-r/--run`, `-u/--umount`, `-c/--cleanup`,
-  `-l/--list-images`, `-i/--inspect`, `-x/--exec`, `-k/--kill`, `-n/--no-nsenter`, `-D/--daemonize`,
+  `-l/--list-images`, `-i/--inspect`, `-x/--exec`, `--kill`, `-n/--no-nsenter`, `-D/--daemonize`,
   `--user`, `--group`, `--hostname`, `--env`, `--env-file`, `-v/--volume`, `--list-volumes`, `--delete-volume`,
   `--delete-volume-full`, `--list-processes`, `--clean-processes`, `-t/--test`, `--log-level`,
   `-h/--help`, `-V/--version`)