Add quick actions to sessions

Rename, AI retitle, pin and archive, reached from the "..." button on
every session row and in the chat header. Sessions carry one lifecycle
state (normal, pinned, archived) rather than independent flags, so the
sidebar groups them without an unrepresentable pinned-and-archived case.

The AI retitle runs as a one-shot against the transient KV cache through
a new CheckpointTarget::OneShot, leaving no session or checkpoint behind
while still reporting as local work in the metrics.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

assets/icons/pin.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="none" stroke="#fff" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12.25 2.75 17.25 7.75M13.5 6.5 8.75 8.25 4.5 12.5l3 3 4.25-4.25 1.75-4.75M6.75 13.25 3 17"/>
+</svg>