feat(config): T-1277 — detach, and a failed job that looked busy

core/process.py spawns a child that outlives its parent: its own session, so a
signal to the parent's group or a timeout kill does not take the work with it;
re-execing reach by BARE NAME, because an absolute path would freeze the child
to whichever checkout was current at spawn time and silently run the wrong
source after a repoint; and streams kept separate exactly as in the foreground,
events to <id>.jsonl and real output to <id>.out.

Testing a case the ticket did not name found a real hole. Recording completion
inside @command looked right and was wrong: a child that fails BEFORE any
command runs — bad arguments, an unknown verb, an import error — never reaches
that decorator. `reach --detach check bogus` left its metadata reading
"running" forever with the process long gone. That is the exit-0 trap wearing a
new disguise and worse than the original, because a failed job that looks busy
sits somewhere nobody is watching, and a caller polling for completion would
wait indefinitely on something that failed in milliseconds.

So completion is recorded at the PROCESS's exit instead. main.py gains main(),
wrapping cli() in a single try/finally, and the entry point moves to
main:main. Every exit path now passes through one place. Removed from @command
rather than left in both — two writers of one field is how they drift.

Verified on three paths: success records done/0, a real drift failure records
failed/1, and the parse failure that exposed the hole now records failed/2.

One narrow conformance exemption, with its reason inline so it does not read as
an oversight: the no-domain-imports-core.jobs invariant fired on main.py,
correctly by its letter and wrongly by its purpose. main.py is not a command;
it is the entry point, and it already owns --detach.

Still open, and carried to T-1278: a child killed outright cannot record
anything, so jobs list must reconcile against process liveness rather than
trusting the file.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tooling/core/command.py

@@ -18,6 +18,7 @@ will call:
 from __future__ import annotations
 
 import functools
+import os
 from collections.abc import Callable
 from typing import Any, TypeVar
 
@@ -56,7 +57,11 @@ def command(func: F) -> F:
 
     @functools.wraps(func)
     def wrapped(*args: Any, **kwargs: Any) -> Any:
-        token = jobs.begin()
+        # A detached child adopts the id its parent already reported; a
+        # foreground run mints a fresh one.
+        # A detached child adopts the id its parent already reported; a
+        # foreground run mints a fresh one.
+        token = jobs.begin(os.environ.get(jobs.ENV_JOB_ID))
         try:
             return inner(*args, **kwargs)
         finally: