v0.8.9: sessions in the Qt front end, transcript replay, UI test helpers

A conversation is now something the window can pick. The sidebar lists the
workspace's stored sessions and switching replays one into the view.

Sessions (app/src/core/AgentCore.*, app/qml/SessionRow.qml, Sidebar.qml):
- AgentCore::open() resolves the workspace's current session through
  cst_agent_store_resolve_default() before the engine starts. Without it the
  engine opened session.jsonl in the process's working directory, next to a
  session list that stayed empty.
- sessionId property; sessions() (id, title, firstUser, messages, mtime in ms,
  current); newSession(), switchSession(), deleteSession() (moves the engine
  off a session it has open first), renameSession(); sessionChanged and
  sessionListChanged signals. No mutation runs during a turn.
- history() turns the open session back into the rows the live signals build
  (kind/role/text/name/state/detail/output/exitCode/complete): system prompt
  skipped, notes and attachments shown, assistant reasoning as a think row,
  tool_calls_json as one card per call, and each result paired with its call
  the way findTool() pairs the live stream. A call with no result keeps state
  "unfinished" ("no result"), so a replay never shows a spinner that cannot
  stop. Main.qml replays on sessionChanged.
- SessionRow.qml is a file of its own: as an inline component it would not see
  the Repeater's modelData. Session controls are disabled while a turn runs.

Tests (app/src/ui/UiTest.cpp, app/src/main.cpp):
- the fixture session is written through the store's own API (a tool call with
  its result), then the checks click its row, assert the five replayed rows and
  the paired result, start two new sessions (the view empties), delete one by
  its button (the open one stays), and switch back. 28 checks in total.
- item() falls back to the visual tree: delegates a model owns are not always
  in the window's QObject tree, which is why the rows were invisible to
  findChild even though they were on screen.
- --uitest drives a scratch workspace unless --workspace is given; it creates
  and deletes sessions. CST_APP_SHOT=file saves the window as a PNG when the
  run ends and CST_APP_DUMP=1 prints the session rows seen; both are how the
  layout gets looked at on a machine without a screen.

QML: ThinkBlock's click target is a TapHandler on its row (a MouseArea anchored
inside a layout is undefined behavior and Qt said so).

Verified: 28/28 UI checks, --selftest, ctest cst-app-selftest, make check rc=0,
and a screenshot of a replayed session (cards, tool card with its result, the
think block, the sidebar list).

Version 0.8.9.

include/cst_version.h

@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 8
-#define CST_VERSION_PATCH 8
+#define CST_VERSION_PATCH 9
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)