Pinned models that know which side you are on

Reported: a pinned model always opened an ordinary chat, even with Agents
selected in the sidebar. They now carry `&kind=agent` with the switch -- a
preselection like `?model=` itself, so the new-chat screen still decides and
nothing is fixed until the first message is sent.

They sit above the tree the switch swaps, so this is the same shape as the New
chat button a few commits ago and gets the same treatment: their own partial,
arriving out of band. The group is rendered even when nothing is pinned,
because a block that vanished when the last model was unpinned would leave that
fragment with nowhere to land -- and htmx says nothing at all when a target is
missing, which is the silent failure this codebase keeps cataloguing.
`.nav-group--pinned:empty` stops the empty one taking room.

Chasing it turned up something else. The shortcuts came from `_chat_context`,
which only the chat pages build -- so the library, connections, settings and
folder pages carried the sidebar without them. A shortcut that is there on one
page and gone on the next. They come from `sidebar_context` now, where they
belong: it is sidebar content, and it is what the fragment route has.

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

CLAUDE.md

@@ -20,7 +20,7 @@ lembas info                         # paths + counts, useful when confused
 lembas secret-key                   # generate LEMBAS_SECRET_KEY
 lembas create-admin                 # create or promote an admin
 
-pytest                              # 1412 tests, ~87s
+pytest                              # 1419 tests, ~87s
                                     # PLAN.md tracks what is and is not built
 ruff check .                        # lint (line length 100)
 python scripts/build_artwork.py     # regenerate artwork (SVG + PWA icons;
@@ -999,6 +999,17 @@ already been a bug once.
 `position` order. Pinned models get shortcuts in the chat sidebar and nothing
 else -- a picker whose order differs from the admin screen is just confusing.
 
+The shortcuts come from `sidebar_context`, not from `_chat_context` where they
+began: they are sidebar content, the fragment route that re-renders the sidebar
+has only the former, and the library and connections pages carry the sidebar
+without ever calling the latter -- so the shortcuts were simply absent on all of
+them. They carry `&kind=agent` with the switch, and they sit above the tree it
+swaps, so they arrive out of band exactly as the New chat button does. The group
+is rendered **even when empty**, because a block that vanished when the last
+model was unpinned would leave that out-of-band fragment with nowhere to land,
+and htmx says nothing at all when a target is missing; `.nav-group--pinned:empty`
+is what stops the empty one taking room.
+
 **System prompts are precedence, not concatenation.** chat > folder > model >
 instance, most specific wins outright
 (`services/chat.py:effective_system_prompt`). Stacking them reads well in a