feat: drawer branching UI (T98.1)

chat/templates/_drawer.html

@@ -414,6 +414,48 @@
     {% endif %}
   </section>
 
+  <section class="drawer-section">
+    <h3>Branches</h3>
+    {% if branches %}
+      <ul class="branch-list">
+        {% for b in branches %}
+          <li class="branch-row{% if b.is_active %} branch-active{% endif %}">
+            <strong>{{ b.name }}</strong>
+            {% if b.is_active %}<span class="muted"> (active)</span>{% endif %}
+            <span class="muted"> &middot; {{ b.event_count }} events</span>
+            {% if not b.is_active %}
+              <form class="inline-edit"
+                    hx-post="/chats/{{ chat.id }}/drawer/branch/switch"
+                    hx-target="#drawer" hx-swap="innerHTML">
+                <input type="hidden" name="name" value="{{ b.name }}">
+                <button type="submit">Switch</button>
+              </form>
+            {% endif %}
+          </li>
+        {% endfor %}
+      </ul>
+    {% else %}
+      <p class="muted">No branches yet.</p>
+    {% endif %}
+    <details>
+      <summary>Create branch</summary>
+      <form class="inline-edit"
+            hx-post="/chats/{{ chat.id }}/drawer/branch/create"
+            hx-target="#drawer" hx-swap="innerHTML">
+        <label>
+          Name:
+          <input type="text" name="name" required
+                 placeholder="e.g. experiment_a">
+        </label>
+        <label>
+          Origin event id:
+          <input type="number" name="origin_event_id" required min="0">
+        </label>
+        <button type="submit">Create</button>
+      </form>
+    </details>
+  </section>
+
   <section class="drawer-section">
     <h3>Pinned memories ({{ pinned|length }} / {{ pin_cap }})</h3>
     {% if pinned %}

chat/web/drawer.py

@@ -36,7 +36,14 @@ from fastapi.responses import HTMLResponse
 from fastapi.templating import Jinja2Templates
 
 from chat.eventlog.log import append_and_apply
+from chat.services.branching import (
+    branch_from_event,
+    list_branches_with_metadata,
+    switch_active_branch,
+)
+from chat.services.delete_impact import compute_delete_impact
 from chat.services.relationship_seed import seed_inter_bot_edges
+from chat.services.rewind import execute_rewind
 from chat.services.scene_summarize import apply_scene_close_summary
 from chat.state.edges import get_edge
 from chat.state.entities import get_bot, get_you, list_bots
@@ -169,6 +176,11 @@ async def drawer(chat_id: str, request: Request, conn=Depends(get_conn)):
     active_events = list_active_events(conn, chat_id)
     open_threads = list_open_threads(conn, chat_id)
 
+    # T98.1: branch metadata (every chat sees the global branch list — branches
+    # may be chat-scoped or global, so :func:`list_branches_with_metadata`
+    # returns both flavours and the template highlights the active one).
+    branches = list_branches_with_metadata(conn, chat_id)
+
     return TEMPLATES.TemplateResponse(
         request,
         "_drawer.html",
@@ -196,6 +208,7 @@ async def drawer(chat_id: str, request: Request, conn=Depends(get_conn)):
             "pin_cap": PIN_CAP,
             "active_events": active_events,
             "open_threads": open_threads,
+            "branches": branches,
         },
     )
 
@@ -1080,3 +1093,98 @@ async def close_thread(
         },
     )
     return await drawer(chat_id, request, conn)
+
+
+# --- T98.1 branching UI --------------------------------------------------
+#
+# Three POST endpoints wired to the Phase 4 :mod:`chat.services.branching`
+# helpers. The drawer's "Branches" panel exposes:
+#
+# * Create from a free-form ``origin_event_id``.
+# * Switch the active branch by name.
+# * Convenience "branch from this turn" against a per-turn event_id (the
+#   chat surface stamps ``id="turn-<event_id>"`` on every turn so users can
+#   pick the right one without copying ids by hand).
+#
+# All three return the refreshed drawer partial; failures from the service
+# layer (duplicate name, unknown branch, invalid origin) surface as 400 so
+# HTMX displays the inline error.
+
+
+@router.post(
+    "/chats/{chat_id}/drawer/branch/create",
+    response_class=HTMLResponse,
+)
+async def create_branch(
+    chat_id: str,
+    request: Request,
+    name: str = Form(...),
+    origin_event_id: int = Form(...),
+    conn=Depends(get_conn),
+):
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+    try:
+        branch_from_event(
+            conn,
+            name=name,
+            origin_event_id=int(origin_event_id),
+            chat_id=chat_id,
+        )
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    return await drawer(chat_id, request, conn)
+
+
+@router.post(
+    "/chats/{chat_id}/drawer/branch/switch",
+    response_class=HTMLResponse,
+)
+async def switch_branch(
+    chat_id: str,
+    request: Request,
+    name: str = Form(...),
+    conn=Depends(get_conn),
+):
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+    try:
+        switch_active_branch(conn, name=name)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    return await drawer(chat_id, request, conn)
+
+
+@router.post(
+    "/chats/{chat_id}/drawer/branch/from-turn/{event_id}",
+    response_class=HTMLResponse,
+)
+async def branch_from_turn(
+    chat_id: str,
+    event_id: int,
+    request: Request,
+    name: str = Form(...),
+    conn=Depends(get_conn),
+):
+    """Convenience: branch from a specific turn event.
+
+    Identical to :func:`create_branch` except ``origin_event_id`` is
+    encoded in the URL — the chat surface renders one such form per turn
+    so users can fork mid-conversation without authoring an event id by
+    hand.
+    """
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+    try:
+        branch_from_event(
+            conn,
+            name=name,
+            origin_event_id=int(event_id),
+            chat_id=chat_id,
+        )
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    return await drawer(chat_id, request, conn)