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

AGENTS.md

@@ -291,7 +291,10 @@ Every `tools/wikitool` call has exactly four outcomes:
 
 1. **Success (exit 0).** Continue.
 2. **Validation error (exit 1 with an `ERROR` line).** Not transient - re-running unchanged
-   fails identically. Read the message, fix the cause, retry **once** with corrected input.
+   fails identically. The `ERROR` line on stdout is followed by the command's ON FAILURE
+   reaction(s) on stderr - the same text `wikitool <cmd> -h` prints, without a second call -
+   or a bare `see: wikitool <cmd> -h` pointer where the record has none yet. Read the
+   message, fix the cause, retry **once** with corrected input.
 3. **User clearance required (exit 42).** Not an error and not yours to resolve: show the
    command's output to the user verbatim and stop. See [Gates](#gates).
 4. **Unexpected error (timeout, crash, interrupted process).** Do not guess whether it