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

CHANGES.md

@@ -59,7 +59,7 @@ concern - readable here, never shipped as something to parse.
 
 ---
 
-## 7.1.0-beta.21 - 2026-09-26 - Command records: NOTES is always a tuple of bullets; every record's examples are tested
+## 7.1.0-beta.22 - 2026-09-26 - fail() prints the command's ON FAILURE lines on stderr
 
 **Author:** Torben Nehmer
 
@@ -70,6 +70,7 @@ concern - readable here, never shipped as something to parse.
 **Medium impact**
 - CalDAV task-tracker provider (Nextcloud Tasks, iOS Reminders); review reports unknown-value findings instead of skipping them
 - dist export no longer cuts the dist export record out of the shipped tools/CONTRACT.md
+- fail() prints the command's ON FAILURE lines on stderr
 
 **Low impact**
 - version bump no longer points at version release in its output
@@ -342,6 +343,21 @@ over the real registry hold what the rewrite established: every command has at l
 example, and every command with a gate shows how its clearance is passed back in (`--confirm`,
 `--confirm-rebase` or `--resume`).
 
+### fail() prints the command's ON FAILURE lines on stderr
+
+`_util.fail()` used to print only its `ERROR` line; the reaction a caller needs the moment a
+command declines lived one lookup away, in `wikitool <cmd> -h`'s ON FAILURE section - exactly
+the lookup AGENTS.md's own tool error contract already warned is the one most likely to be
+skipped in the heat of a failure. `fail()` now prints that section's exit-1 causes (each with a
+reaction) right after the `ERROR` line, on stderr and as plain text rather than through Rich - a
+reaction can carry a literal `[--flag]`, which Rich would otherwise read as markup. A record
+with no exit-1 cause of its own falls back to a bare `see: wikitool <cmd> -h` pointer;
+`docs contract` is the one real command that hits it today. Nothing about stdout changes: a
+command's output on success, or up to and including its `ERROR` line on failure, is
+byte-identical to before. `cli.py`'s and `_util.py`'s lookup of the running command's
+`cli_contract` path is now one shared function, `cli_contract.path_of`, in place of a private
+copy that used to live only in `cli.py`.
+
 ---
 
 ## 7.0.0 - 2026-09-22 - Task-Tracker-Anbindung: Vorhaben als Seitenart, Verpflichtungsschicht, Weekly Review als Read-Time-Join