fix(brain-install): remove dead-branch mapping; add catch-all to filter case

Two code-quality fixes after review of dc71e23 (Task 1):

I1 — Dead-branch logic in install.sh:
The plugin-install block sits inside the `elif [ "$mode" = "user" ]` branch,
so `[ "$mode" = "project" ] && plugin_target="consumer"` is unreachable
(`$mode` is always "user" here). Replaced with hardcoded `--install-target=user`
plus clarifying comment about future consumer-mode wiring.

I2 — Filter case missing catch-all in install-plugins.sh:
The two `case "$install_target"` blocks (validation at lines 40-43, filter at
lines 75-82) were physically separate. If a future enum value is added to
validation without updating filter, $filter would be unset → set -u crash.
Added explicit `*)` catch-all with log_error + exit 1.

Plan §Task 1 Step 1.7 contained the same I1 bug as dc71e23 — implementer
followed plan faithfully. This fix corrects the code; plan stays as historical
record. Future plans referencing this pattern should match THIS fix, not the
original plan text.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/lib/install-plugins.sh

@@ -79,6 +79,10 @@ case "$install_target" in
     consumer)
         filter='.plugins | to_entries[] | select(.value[0].target == "consumer" or .value[0].target == "user-level+consumer") | .key'
         ;;
+    *)
+        log_error "internal: unhandled install_target=$install_target (should have been validated above)"
+        exit 1
+        ;;
 esac
 
 for plugin in $(jq -r "$filter" "$manifest"); do