Read hook payload from fd 0, not /dev/stdin

Claude Code delivers the hook payload on a socket. Opening it by path
('/dev/stdin' -> /proc/self/fd/0) fails with ENXIO, so bash-guard.mjs threw on
every invocation and its bare catch exited 0 silently. The guard looked like it
was never dispatched; it was dying on line 29 each time.

readFileSync(0) is read() on the descriptor with no open(), which works on a
socket. The catch now logs instead of swallowing, so this failure mode can never
again masquerade as non-dispatch.

Adds test-bash-guard.py, which drives the guard over a socketpair. A pipe would
not reproduce the bug, so the socket is load-bearing. Verified failing against
the pre-fix guard (silent, no output) and passing after.

Removes the five diagnostic probes and probe.mjs; they served their purpose.
Bumps to 1.0.2 because the plugin cache is keyed by version and an unchanged
version silently skips reinstall.

plugins/guards/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "guards",
-  "version": "1.0.1",
+  "version": "1.0.2",
   "description": "Personal enforcement hooks: jj-only version control, no Claude attribution, build+test gate on push, and secret scrubbing on file writes.",
   "author": {
     "name": "Malcolm Roberts"