pi_wake_agent: --pane targeting + crontab backend fallback + cronicle HCL fix + hermetic tests

- --pane/--pane-id: write-chars AND write(13) hit terminal_1 (bottom pane)
- cronicle_available()=shutil.which + daemon-live check; crontab/crnd auto-fallback
  (cronicle daemon down here -> live scheduler is system crnd)
- fix cronicle_install invalid HCL (command=[...] array, was bare comma-list)
- rewrite TestCrontabEntry hermetic (subprocess mocked; real crontab never touched)
- add --pane-id / no-pane omission / HCL-array mutation-litmus tests; .sh --pane
- cronicle.hcl: corrected pi_wake_pi_test_20m (msg + --pane terminal_1), valid HCL
- live 20m crontab doorbell installed for pi_test/terminal_1, bell verified delivered
- SIGQUIT rescued a locked pi (final_test @1318s); steer queued + bus-posted

cronicle.hcl

@@ -0,0 +1,24 @@
+# pi_wake_agent schedules (cronicle backend).
+#
+# IMPORTANT (see AGENTS.md): on this host the cronicle daemon is NOT running
+# (not supervised), so these schedules are INERT. The LIVE scheduler is system
+# crnd via crontab (see `crontab -l`). This HCL is only served when
+# `cronicle run --path <this-file>` is supervised. The pi_test 20m wake is
+# installed into crontab as well, so it actually fires regardless.
+
+schedule "pi_wake_pi_test_20m" {
+  cron = "@every 20m"
+
+  task "wake" {
+    command = ["/mnt/dolphinng5_predict/pi_wake_agent.py", "--run", "--interval", "20m", "--sessions", "pi_test", "--pane", "terminal_1", "--msg", "Please carefully continue your work"]
+  }
+}
+
+# Sample multi-session schedule (kept from the original config).
+schedule "pi_wake_s1_s2_1h" {
+  cron = "@every 1h"
+
+  task "wake" {
+    command = ["/mnt/dolphinng5_predict/pi_wake_agent.py", "--run", "--interval", "1h", "--sessions", "s1,s2", "--msg", "test"]
+  }
+}