Compaction: a button, and automatically when the window fills

A long conversation eventually just stops working. Compaction summarises the
earlier turns and sends the summary in their place.

The messages are kept. They stay in the transcript behind a collapsed
divider and simply stop being part of the request, which is what makes the
button safe to press and automatic compaction safe to have at all: a summary
that came out badly is a bad turn, not a lost conversation.

Stored on the Chat, not as a synthetic Message. A synthetic row needs a
role -- `system` breaks the one-system-message rule the moment build_messages
emits it beside the harness, and user/assistant makes it a turn people can
edit, regenerate from and copy, indistinguishable from a real one in all
four places a bubble is rendered. Worse, "editing rewinds, it does not
branch" would silently delete it and leave no marker that compaction had
happened at all.

The summary goes out as a user turn and an assistant turn, not one. A
leading assistant breaks templates requiring the first non-system message to
be user; a lone leading user produces user, user whenever the kept history
starts on a user turn -- which it always does, because the cutoff lands on a
finished reply.

compacted_through_id is a plain id rather than a foreign key: migrations.py
compiles only the column type, so a REFERENCES clause would exist on a fresh
database and not on an upgraded one, and a constraint half the fleet has is
worse than none. cutoff_message validates it on every read instead, and a
rewind past the boundary clears it.

Compacting again summarises only the delta, with the previous summary
supplied to be subsumed. Re-summarising the whole chat each time grows
quadratically and eventually exceeds the window it is protecting.

Automatically at the top of _run, not in post_message: that route's contract
is to return immediately and leave the slow part to a resumable connection,
and it also means build_request is called once, after compaction, with no
second assembly path. The trigger is the last reply's recorded usage plus an
estimate of the new turn -- retrospective because true prompt_tokens are only
knowable after a response, plus the delta because otherwise fifty thousand
characters pasted into the composer overflow a window that read 90% last
turn. It never fires when the context length is unknown. It does fire on
estimated counts, which is safe here precisely because nothing is lost.

_maybe_compact never raises: a failure logs and sends the uncompacted
request. A `status` event says "Summarising earlier messages…" in the
meantime, because a silent multi-second pause before the first token is what
a hang looks like.

The wording is three fragments under Admin - Prompts. Clearing task.compact
turns compaction off entirely.

Also adds compaction.moment(): SQLite does not store the offset, so a row
loaded from disk is naive while one in the session's identity map keeps its
tzinfo, and comparing the two raises. Every comparison here is between
exactly those.

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

src/lembas/services/prompts.py

@@ -156,6 +156,16 @@ VARIABLES: tuple[Variable, ...] = (
     ),
     Variable("question", "Question", "The first message. Chat title task only."),
     Variable("answer", "Answer", "The first reply. Chat title task only."),
+    Variable(
+        "transcript",
+        "Transcript",
+        "The turns being summarised, oldest first. Compaction task only.",
+    ),
+    Variable(
+        "previous_summary",
+        "Earlier summary",
+        "The summary from a previous compaction, if there was one. Compaction task only.",
+    ),
 )
 
 VARIABLE_NAMES = frozenset(variable.name for variable in VARIABLES)
@@ -764,6 +774,77 @@ BUILTIN: tuple[Fragment, ...] = (
             "Assistant: {{answer}}"
         ),
     ),
+    Fragment(
+        key="task.compact",
+        label="Compaction summary",
+        group=GROUP_TASKS,
+        order=410,
+        variables=("transcript", "previous_summary"),
+        hint="A separate one-message request, not part of any chat. Clear it to "
+        "turn compaction off entirely: the button says so and nothing is "
+        "summarised automatically.",
+        default=(
+            "Summarise the conversation below so it can be carried forward after the "
+            "earlier turns are dropped from your context. This is a working record, "
+            "not a report for a reader.\n"
+            "\n"
+            "Keep, under these headings and in this order:\n"
+            "\n"
+            "## What we are doing\n"
+            "The goal, and where we have got to.\n"
+            "\n"
+            "## Decisions\n"
+            "Anything settled, and why. A decision without its reason gets argued "
+            "again.\n"
+            "\n"
+            "## Facts established\n"
+            "Names, numbers, versions, file paths, URLs and identifiers, copied "
+            "exactly. Do not round them, paraphrase them or reconstruct one from "
+            "memory — if it is not in the transcript, leave it out.\n"
+            "\n"
+            "## Open threads\n"
+            "What is unfinished, and what was about to happen next.\n"
+            "\n"
+            "Leave out pleasantries, retracted ideas and anything already superseded. "
+            "Do not answer the conversation: you are recording it. Write in the "
+            "language of the conversation, and stay under 500 words.\n"
+            "\n"
+            "{{previous_summary}}\n"
+            "\n"
+            "## Transcript\n"
+            "\n"
+            "{{transcript}}"
+        ),
+    ),
+    Fragment(
+        key="task.compact_lead",
+        label="How a summary is introduced",
+        group=GROUP_TASKS,
+        order=420,
+        hint="Sits in front of the summary, in the turn that replaces the "
+        "messages no longer being sent. Without it a model reads the summary as "
+        "something the person has just typed.",
+        default=(
+            "Here is a summary of the earlier part of this conversation. Those "
+            "messages are no longer in your context. Treat this summary as an "
+            "accurate record of them and rely on it rather than on what you can no "
+            "longer see; if it does not cover something you need, say so instead of "
+            "filling the gap."
+        ),
+    ),
+    Fragment(
+        key="task.compact_ack",
+        label="The model's acknowledgement",
+        group=GROUP_TASKS,
+        order=430,
+        hint="One assistant turn after the summary, so the conversation still "
+        "alternates user, assistant, user. Several chat templates reject a "
+        "history that does not.",
+        default=(
+            "Understood. I have the summary of the earlier turns and will carry on "
+            "from there."
+        ),
+    ),
 )
 
 register_source(_builtin_source)