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

src/pid_file.cpp

@@ -103,3 +103,53 @@ void release_session_lock(const SessionLock& lock) {
         spdlog::warn("failed to remove session pid file {}: {}", lock.path.string(), ec.message());
     }
 }
+
+std::vector<SessionInfo> list_sessions() {
+    std::vector<SessionInfo> sessions;
+
+    auto dir = session_run_dir();
+    std::error_code dir_ec;
+    if (!std::filesystem::is_directory(dir, dir_ec)) {
+        return sessions;
+    }
+
+    std::error_code it_ec;
+    for (auto it = std::filesystem::directory_iterator(dir, it_ec);
+         !it_ec && it != std::filesystem::directory_iterator(); it.increment(it_ec)) {
+        const auto& path = it->path();
+
+        int fd = open(path.c_str(), O_RDWR);
+        if (fd < 0) {
+            spdlog::debug("failed to open session file {}: {}", path.string(), strerror(errno));
+            continue;
+        }
+
+        char buf[32] = {};
+        ssize_t n = read(fd, buf, sizeof(buf) - 1);
+        pid_t pid = n > 0 ? static_cast<pid_t>(std::atoi(buf)) : 0;
+
+        bool running = true;
+        if (flock(fd, LOCK_EX | LOCK_NB) == 0) {
+            running = false;
+            flock(fd, LOCK_UN);
+        }
+        close(fd);
+
+        if (pid <= 0) {
+            spdlog::debug("skipping malformed session pid file {}", path.string());
+            continue;
+        }
+
+        std::string filename = path.filename().string();
+        std::string suffix = fmt::format("-{}", pid);
+        std::string container_name =
+            filename.size() > suffix.size() &&
+                    filename.compare(filename.size() - suffix.size(), suffix.size(), suffix) == 0
+                ? filename.substr(0, filename.size() - suffix.size())
+                : filename;
+
+        sessions.push_back(SessionInfo{pid, container_name, running});
+    }
+
+    return sessions;
+}