feat: add kugetsu session manager skill

- skills/kugetsu/SKILL.md: Agent skill documentation following agentskills.io spec
- skills/kugetsu/scripts/kugetsu: Shell wrapper for opencode session management
  - Commands: start, list [--all], resume, stop, help
  - State tracking: used → idle (graceful) or left (interrupted)
  - Auto-fill message on resume
  - Confirmation prompt when resuming used session
- skills/kugetsu/scripts/kugetsu-install.sh: Installation script for users

Implements Phase 1 of issue #11 - basic session management layer
for remote agent control without Hermes dependency.

skills/kugetsu/scripts/kugetsu-install.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+set -euo pipefail
+
+KUGETSU_DIR="${KUGETSU_DIR:-$HOME/.kugetsu}"
+BIN_DIR="$KUGETSU_DIR/bin"
+
+echo "Installing kugetsu to $KUGETSU_DIR..."
+
+mkdir -p "$BIN_DIR"
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+cp "$SCRIPT_DIR/kugetsu" "$BIN_DIR/kugetsu"
+chmod +x "$BIN_DIR/kugetsu"
+
+add_to_shell() {
+    local rc_file="$1"
+    local export_line="export PATH=\"\$HOME/.kugetsu/bin:\$PATH\""
+    
+    if [ -f "$rc_file" ]; then
+        if grep -q "$export_line" "$rc_file" 2>/dev/null; then
+            echo "$rc_file already has kugetsu in PATH"
+        else
+            echo "" >> "$rc_file"
+            echo "# kugetsu - opencode session manager" >> "$rc_file"
+            echo "$export_line" >> "$rc_file"
+            echo "Added to $rc_file"
+        fi
+    else
+        echo "" >> "$rc_file"
+        echo "# kugetsu - opencode session manager" >> "$rc_file"
+        echo "$export_line" >> "$rc_file"
+        echo "Created $rc_file with kugetsu PATH"
+    fi
+}
+
+add_to_shell "$HOME/.bashrc"
+add_to_shell "$HOME/.zshrc"
+
+echo ""
+echo "Installation complete!"
+echo ""
+echo "Run this to start using kugetsu immediately:"
+echo "    export PATH=\"\$HOME/.kugetsu/bin:\$PATH\""
+echo ""
+echo "Or start a new shell."
+echo ""
+echo "Usage:"
+echo "    kugetsu start <session_id> <message>   Start a new session"
+echo "    kugetsu list                          List sessions"
+echo "    kugetsu resume <session_id> [msg]     Resume a session"
+echo "    kugetsu stop <session_id>             Stop a session"
+echo "    kugetsu help                          Show help"