feat(mem0-plugin): add PostToolUseFailure, PostCompact, SessionEnd, SubagentStop hooks

Close 4 lifecycle coverage gaps:
- PostToolUseFailure: catch failed mem0 MCP calls, classify errors, inject retry guidance
- PostCompact: inject parallel search commands for context recovery after compaction
- SessionEnd: last-chance transcript capture with dedup against Stop hook
- SubagentStop: remind parent agent to persist subagent learnings

Also patch on_stop.sh to write capture marker for SessionEnd dedup.

mem0-plugin/scripts/on_subagent_stop.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# Hook: SubagentStop
+#
+# Fires when a subagent finishes. Injects a reminder to capture any
+# learnings the subagent produced that the parent agent should store.
+#
+# Input:  JSON on stdin with agent_type, result_summary
+# Output: Context injected into parent agent's context (exit 0)
+
+set -uo pipefail
+
+if [ -n "${MEM0_DEBUG:-}" ]; then
+  mkdir -p "$HOME/.mem0" && exec 2>>"$HOME/.mem0/hooks.log"
+fi
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+
+INPUT=$(cat)
+AGENT_TYPE=$(echo "$INPUT" | jq -r '.agent_type // ""' 2>/dev/null || echo "")
+RESULT_SUMMARY=$(echo "$INPUT" | jq -r '.result_summary // ""' 2>/dev/null || echo "")
+
+# Skip short/empty results — nothing worth capturing
+if [ ${#RESULT_SUMMARY} -lt 50 ]; then
+  exit 0
+fi
+
+# Skip explorer/plan agents — read-only, rarely produce storable learnings
+case "$AGENT_TYPE" in
+  Explore|Plan)
+    exit 0
+    ;;
+esac
+
+if [ -z "${MEM0_API_KEY:-}" ]; then
+  exit 0
+fi
+
+. "$SCRIPT_DIR/_identity.sh" 2>/dev/null || true
+
+cat <<EOF
+
+## Subagent completed: $AGENT_TYPE
+
+Review the subagent result for learnings worth persisting to mem0.
+If the subagent discovered something reusable (a fix, pattern, decision, or anti-pattern),
+store it via \`add_memory\` with appropriate metadata type.
+
+Only store if genuinely valuable — skip trivial subagent results.
+EOF
+
+exit 0