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/SKILL.md

@@ -0,0 +1,125 @@
+---
+name: kugetsu
+description: Session manager for opencode CLI. Use when managing long-running opencode sessions, resuming interrupted work, or tracking session state across disconnects. Features state tracking (used/idle/left), auto-fill last message on resume, and safe locking via confirmation prompts.
+license: MIT
+compatibility: Requires opencode CLI, bash, and filesystem access for session state.
+metadata:
+  author: shoko
+  version: "1.0"
+---
+
+# kugetsu - OpenCode Session Manager
+
+Manages opencode CLI sessions with state tracking and safe resume.
+
+## Installation
+
+### For Human Users
+Run once on a new host:
+```bash
+. skills/kugetsu/scripts/kugetsu-install.sh
+```
+
+### For Agents (Self-Install)
+Copy the script to your PATH:
+```bash
+cp skills/kugetsu/scripts/kugetsu ~/.local/bin/kugetsu
+chmod +x ~/.local/bin/kugetsu
+```
+
+Or source directly when needed:
+```bash
+. skills/kugetsu/scripts/kugetsu
+```
+
+## Session State
+
+| State | Meaning | Resumable? |
+|-------|---------|------------|
+| `used` | Session is active (process running) | Yes (with confirmation) |
+| `idle` | Session ended gracefully | No |
+| `left` | Session interrupted/crashed | Yes |
+| `invalid` | Session data missing/corrupt | No |
+
+## Session Directory
+
+Sessions are stored in `~/.kugetsu/sessions/<session_id>/`:
+- `state` - current state (used/idle/left/invalid)
+- `message` - last user message (for auto-fill)
+- `pid` - active process PID (when used)
+
+## Commands
+
+### kugetsu start `<session_id>` `<message>`
+Start a new session:
+```bash
+kugetsu start mytask "fix bug #1"
+```
+- Creates session directory
+- Sets state to `used`
+- Stores PID and message
+- Runs: `opencode run --session <session_id> <message>`
+
+### kugetsu list [--all]
+List sessions:
+```bash
+kugetsu list          # Shows only `left` (resumable)
+kugetsu list --all    # Shows all states
+```
+
+### kugetsu resume `<session_id>` [message]
+Resume an interrupted session:
+```bash
+kugetsu resume mytask                 # Auto-fills last message
+kugetsu resume mytask "continue"     # Uses provided message
+```
+- If state is `used`: prompts for confirmation (someone else might be using)
+- If state is `idle`: errors (not resumable)
+- If state is `left`: proceeds with message
+
+### kugetsu stop `<session_id>`
+Stop a session gracefully:
+```bash
+kugetsu stop mytask
+```
+- Sends SIGTERM to process
+- Sets state to `idle`
+
+### kugetsu help
+Show usage help.
+
+## State Transitions
+
+```
+start ──────────────► used ──────► idle (stop/SIGTERM)
+                         │
+                         └──────► left (kill/SIGINT/crash)
+```
+
+## Example Workflow
+
+```bash
+# Start a long-running task
+kugetsu start issue42 "implement feature X"
+
+# ... time passes, connection drops ...
+
+# Check what sessions are resumable
+kugetsu list
+
+# Resume with auto-filled message
+kugetsu resume issue42
+
+# Later, when done
+kugetsu stop issue42
+```
+
+## Without kugetsu
+
+If kugetsu is not installed, use opencode directly:
+```bash
+opencode run --session mytask "task description"
+opencode run --continue --session mytask "continue"
+opencode session list
+```
+Tradeoff: No state tracking, no auto-fill, no filtered list, no confirmation prompts.