fix: harden the install / update / uninstall scripts

Four things found while re-reading the family of scripts around the
logout fix.

Both installers piped curl straight into tar, so a truncated download
half-extracted — and the installer explicitly supports reinstalling over
an existing install, which turned an interrupted download into a tree
mixing old and new files with no error saying so. They now download to a
temp file and verify the archive in a staging dir before writing
anything to the install directory: the ordering update.sh has had since
it was written, for the same reason.

update.sh never removed files deleted upstream. Extracting over the
install dir only adds and overwrites, so a renamed page under docs/ kept
being mounted read-only into every container for the assistant to read,
and a removed command kept being discovered. It now prunes, from the
directories the tarball owns end to end (web, commands, skills, docs),
whatever the already-verified staging copy does not have. Pruning after
the extraction rather than replacing the directory keeps every
intermediate state a complete install. agents/ is deliberately excluded:
dropping in an agent is a documented extension point, so that directory
is not ours alone and pruning it would delete somebody's work.

uninstall.sh fed `docker ps -aq --filter 'name=skald-'` to `docker rm
-f`. Docker's name filter is a regex matched anywhere in the name, not a
prefix, so any unrelated container merely containing "skald-" was
force-removed. Anchored to ^skald-.

uninstall.sh also matched uname's raw Linux/Darwin while its three
siblings normalize to lowercase. It was correct on its own, but being
the odd one out of four copy-paste relatives is precisely how update.sh
acquired its no-op case arms, so it now normalizes like the others.

Finally, the uninstaller reports that lingering is still enabled and how
to turn it off, rather than disabling it: it is a persistent per-user
setting other user services may rely on by now, so taking it back
silently would stop those too.

update.sh

@@ -296,6 +296,31 @@ main() {
         exit 1
     fi
 
+    # ── Drop files the new build no longer ships ───────────────────────────────
+    # Extracting over the install dir only ever adds and overwrites, so anything
+    # deleted upstream survives forever: a renamed doc page keeps being mounted
+    # read-only into every container for the assistant to read, a removed command
+    # keeps being discovered. For the directories the tarball owns end to end we
+    # therefore prune whatever the (already verified) staging copy does not have.
+    #
+    # Pruned AFTER extracting rather than by replacing the directory, so every
+    # intermediate state is a complete install and the only files removed are
+    # ones the new build has verifiably dropped.
+    #
+    # agents/ is deliberately not in this list: adding an agent is a documented
+    # extension point (agents/<id>/meta.json + AGENT.md), so the directory is not
+    # ours alone and pruning it would delete somebody's work — at the price of an
+    # upstream-deleted agent lingering. bin/ is out too: two files, both
+    # overwritten every time, nothing to reclaim.
+    for owned in web commands skills docs; do
+        [ -d "$STAGING/$owned" ] && [ -d "${INSTALL_DIR}/$owned" ] || continue
+        ( cd "${INSTALL_DIR}/$owned" && find . -type f ) | while IFS= read -r rel; do
+            rel="${rel#./}"
+            [ -e "${STAGING}/${owned}/${rel}" ] || rm -f "${INSTALL_DIR}/${owned}/${rel}"
+        done
+        find "${INSTALL_DIR}/$owned" -mindepth 1 -type d -empty -delete 2>/dev/null || true
+    done
+
     # Update version file for release channel
     if [ "$CHANNEL" = "release" ]; then
         echo "$VERSION" > "$INSTALL_DIR/.release-version"