Improve fm-shim-backend security and D-Bus error handling

AGENTS.md

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+# Agent Guidelines for security-misc
+
+## fm-shim
+
+### D-Bus name acquisition
+
+The fm-shim-backend intentionally uses `DBUS_NAME_FLAG_REPLACE_EXISTING`
+**without** `DBUS_NAME_FLAG_DO_NOT_QUEUE`. When the name cannot be acquired,
+the process warns and omits `READY=1` so that systemcheck alerts the user.
+Hard-failing (via `errx`) removes the service entirely, which is worse than
+running in a degraded-but-monitored state. Do not change this to a fatal error.
+
+### Systemd sandboxing on services that spawn user-facing apps
+
+Do not add restrictive systemd sandboxing directives (`ProtectHome`,
+`MemoryDenyWriteExecute`, `ProtectSystem=strict`, etc.) to the fm-shim
+service. These restrictions are inherited by child processes — the file
+manager and anything it launches. `MemoryDenyWriteExecute` breaks JIT,
+`ProtectHome=read-only` breaks normal file manager operations, etc.
+
+### Closing inherited file descriptors in forked children
+
+Use the `close_range(3, ~0U, CLOSE_RANGE_UNSHARE)` syscall (requires
+`#define _GNU_SOURCE` and `#include <linux/close_range.h>`). Do not iterate
+`/proc/self/fd` manually.
+
+### D-Bus error replies
+
+When sending D-Bus error replies in multiple places, extract a shared helper
+function (e.g. `send_error_message_maybe()`) rather than duplicating the
+pattern inline.