fix(shell): resolve all shellcheck warnings across scripts

- SC2088: replace ~ with $HOME in quoted strings (doctor.sh)
- SC2010/SC2012: replace ls|grep with compgen -G globs (detect-plugins.sh)
- SC2034: remove unused PKG and RED variables (install-plugins.sh, update-all.sh)
- SC2015: convert A&&B||C to proper if/then/else (update-all.sh, install-plugins.sh, session-start.sh)
- SC1090: add shellcheck source directive (statusline.sh)
- SC2129: group redirects into single block (install-plugins.sh)

0 warnings remaining (3 SC1091 info-level expected).

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

hooks/statusline.sh

@@ -21,6 +21,7 @@ BRANCH_STR="${BRANCH:+ ($BRANCH)}"
 # Plan detection (reuse shared lib)
 _lib="$(dirname "${BASH_SOURCE[0]}")/../lib/detect-plugins.sh"
 if [ -f "$_lib" ]; then
+  # shellcheck source=../lib/detect-plugins.sh
   source "$_lib"
   PLAN=$(detect_plan 2>/dev/null || echo "pro")
 else