fix(issue-8,7): harden install_state merge and pin sibling-workspace edge cases

Round-1 review of 4176d75 (planner-reviewer-claude-01, non-blocking):

- update.sh: merge install_state by exact key match (awk, no regex
  interpretation of the key), skip lines without '=', and terminate a fresh
  install_state that lacks a final newline before appending
- tests (#7): an agent-less pane left behind in a later same-cwd workspace
  is reused instead of splitting or creating; MAM_WS_LABEL renames only the
  workspace that was chosen

Refs #8 #7

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

deploy/update.sh

@@ -238,10 +238,20 @@ if [ $HAS_MAM -eq 1 ]; then
     # The failure path (restore_on_failure) still puts all three back verbatim.
     if [ -f ".mam.update-tmp/install_state" ]; then
       touch .mam/install_state
+      # A fresh install_state without a final newline would glue the first
+      # merged key onto its last line.
+      if [ -s .mam/install_state ] && [ -n "$(tail -c1 .mam/install_state)" ]; then
+        printf '\n' >> .mam/install_state
+      fi
       while IFS= read -r line || [ -n "$line" ]; do
+        case "$line" in
+          *=*) ;;
+          *) continue ;;   # not a key=value line
+        esac
         key="${line%%=*}"
         [ -n "$key" ] || continue
-        if ! grep -q "^${key}=" .mam/install_state 2>/dev/null; then
+        # exact key match, no regex interpretation of the key
+        if ! awk -F= -v k="$key" '$1 == k { found = 1 } END { exit !found }' .mam/install_state; then
           printf '%s\n' "$line" >> .mam/install_state
         fi
       done < .mam.update-tmp/install_state