Fix crash when running codeman with no args under bash nounset

[shabo]

Summary

Fixes macOS default Bash (3.2) compatibility issues and improves Discord/Slack notifications so they work in real interactive usage.

What changed

• Fixed set -u crash when running codeman with no args (empty-array expansion).
• Replaced mapfile usage (not available in Bash 3.2) with portable while read loops.
• Added bats regression tests plus CI matrix (Ubuntu + macOS) and expanded ShellCheck coverage.
• Added per-assistant-message notifications (opt-in via CODEMAN_NOTIFY_ON=...,message).
• Session monitoring now respects CODEX_HOME (defaults to ~/.codex).
• Bumped version to v0.1.3.

Why

• Bash 3.2 treats expanding an empty array under set -u as an error (unbound variable).
• Bash 3.2 does not implement mapfile, so codeman l3 ... could fail.
• The original notification logic only emitted on task_complete / approval waits; interactive usage needs message-level pings.

ASCII flow

+-------------------+
| start (set -euo) |
+-------------------+
|
v
+-------------------+
| parse args/flags |
+-------------------+
|
v
+-------------------------------+
| run codex + tail session log |
| (from $CODEX_HOME/sessions) |
+-------------------------------+
|
v
+-----------------------------------------------+
| on new session lines: |
| - wait: denial markers -> Discord/Slack ping |
| - message: agent_message -> Discord/Slack ping |
| - complete: task_complete -> Discord/Slack ping|
+-----------------------------------------------+