tools: fail() prints ON FAILURE lines on stderr after ERROR (#143)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

Files changed:
- AGENTS.md
- CHANGES.md
- VERSION
- tools/CONTRACT.md
- tools/chemenu/cli.py
- tools/chemenu/cli_contract.py
- tools/chemenu/commands/_util.py
- tools/chemenu/tests/test_cli.py
- tools/chemenu/tests/test_cli_contract.py
- tools/chemenu/tests/test_util.py

tools/CONTRACT.md

@@ -74,6 +74,11 @@ tools/wikitool <command> -h
 way, for a human or an agent. Bare `tools/wikitool -h` prints the index below plus a pointer back
 to this form.
 
+A call that ends through `_util.fail()` (exit 1) prints its `ERROR` line to stdout as before, then
+its record's ON FAILURE reaction(s) to stderr, in the same `<cause> -> <reaction>` form `-h` prints
+- so the reaction is in front of the caller without a second `-h` call. A record with no exit-1
+cause of its own falls back to a bare `see: wikitool <cmd> -h` pointer.
+
 ## Commands
 
 <!-- wikitool:commands -->