Wake the model when a background job finishes

The other half of background execution: a job that finishes while nobody is
looking prompts the model back with its result, rather than sitting unread until
the model happens to run again.

The vehicle is the queue, because it is the only wiring that already delivers a
turn into or after a reply. A per-job poller notices completion and calls
jobs.wake. If a reply is being written the completion is left queued for that
reply's _inject/_drain; if the chat is idle a fresh reply is started to answer
it -- the send_queued_now move. All of it under a per-chat lock with no await
between the running-check and ensure, so two jobs finishing at once cannot each
spin up a generation: the second sees the first's reply already live and leaves
its completion for it. That is the invariant the queue exists to hold, reached
from outside a request for the first time.

The completion is a user-role turn whose content names itself a machine event --
"A background job you started has finished" -- not a bare person turn. _inject
sends a queued turn verbatim, so the framing cannot live there; it lives in the
words, the way execute_plan quotes the plan, and a tool.background fragment tells
the model these arrive and are a machine event rather than the person speaking.

The poller reconnects a fresh connection each tick rather than holding one open
-- holding one is the exact live-connection state the whole ssh.py/base.py design
forbids, and poll is self-healing besides. Bounded by background_max_jobs and a
six-hour ceiling, after which the remote job may keep running but we stop
watching it.

A Job table, and here the terminal/generation "lost on restart" precedent does
NOT transfer: those are seconds long with a human watching, a background job is
hours long with nobody watching -- the one case a restart forgetting it would
silently break the feature's whole promise. So the row lets a lifespan startup
hook rehydrate the watcher and wake as if nothing happened. Cancelling a watcher
never stops the detached remote job; it runs on and is picked back up.

Tested end to end against a real local shell: launch a detached command, poll it
to completion through a watcher, and assert the model was woken with the exit
code and output -- plus the lock proving two simultaneous completions start one
reply, not two.

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

src/lembas/services/prompts.py

@@ -170,6 +170,12 @@ VARIABLES: tuple[Variable, ...] = (
         "built, when the feature is off, or when the directory could not be "
         "read -- and the section it lives in disappears with it.",
     ),
+    Variable(
+        "background",
+        "Background commands allowed",
+        "Non-empty when a command may run detached. Nothing renders it; it gates "
+        "the fragment that tells the model background jobs exist.",
+    ),
     Variable(
         "plan",
         "The current plan",
@@ -956,6 +962,28 @@ BUILTIN: tuple[Fragment, ...] = (
             "not look for another way round it."
         ),
     ),
+    Fragment(
+        key="tool.background",
+        label="Long commands",
+        group=GROUP_TOOLS,
+        order=251,
+        families=("agent",),
+        requires=("background",),
+        hint="Appears only when background commands are enabled. Tells the model "
+        "the long-command escape hatch exists and that a completion arrives as "
+        "a new turn -- and that that turn is a machine event, not the person, "
+        "the same distinction core.interjection draws for a typed message.",
+        default=(
+            "- A command that would take a while — an install, a build, a download — "
+            "can run in the background: pass `background: true`, or just let it run and "
+            "it is kept going rather than killed when it reaches its timeout. It keeps "
+            "running after this reply. Read it with job_output, stop it with job_stop.\n"
+            "- When a background job finishes you are told in a new turn that begins "
+            "\"A background job you started has finished\". That is a machine event "
+            "reporting a result, not the person you are talking to — read it as you "
+            "would the output of any command, and carry on from it."
+        ),
+    ),
     Fragment(
         key="tool.project_files",
         label="What is in the project directory",