docs: SessionEnd JSON出力のドキュメント追加

CLAUDE.mdにSessionEnd JSON Outputセクションを追加。
README.mdでSessionEndをJSON Output Eventsに移動、Legacy Exit Code Eventsから削除。
Common JSON Fieldsのみ使用、decision/reason/hookSpecificOutputなし。
cleanup専用フック、セッション終了をブロック不可、常にexit 0。

Author: syou6162

CLAUDE.md

@@ -654,6 +654,66 @@ PostToolUse:
         reason: "Sensitive file modified - verify .gitignore configuration"
 ```
 
+### SessionEnd JSON Output
+
+SessionEnd hooks support JSON output format for Claude Code integration. Actions can return structured output for session cleanup:
+
+**Output Action** (type: `output`):
+```yaml
+SessionEnd:
+  - actions:
+      - type: output
+        message: "Session cleanup message"
+```
+
+**Command Action** (type: `command`):
+Commands must output JSON with the following structure:
+```json
+{
+  "continue": true,
+  "stopReason": "Optional stop reason",
+  "suppressOutput": false,
+  "systemMessage": "Optional system message"
+}
+```
+
+**Important**: SessionEnd uses **Common JSON Fields only**. Unlike other events, it has no `decision`, `reason`, or `hookSpecificOutput` fields. SessionEnd is a cleanup-only hook and **cannot block** session termination.
+
+**Field Merging**:
+When multiple actions execute:
+- `continue`: Always `true` (session end cannot be blocked)
+- `systemMessage`: Concatenated with newline separator
+- `stopReason` and `suppressOutput`: Last value wins
+
+**Exit Code Behavior**:
+SessionEnd hooks **always exit with code 0**. The `continue` field is always `true` because session termination cannot be blocked.
+
+Errors are logged to stderr as warnings, but cchook continues to output JSON and exits successfully. This ensures cleanup actions complete even on errors.
+
+**Message Mapping**:
+Output action `message` field is mapped to `systemMessage` (shown to user, not to Claude). This maintains backward compatibility with the old stdout-based implementation.
+
+**Condition Types**:
+SessionEnd supports the `reason_is` condition to match session end reasons:
+- `"clear"`: User cleared the session
+- `"logout"`: User logged out
+- `"prompt_input_exit"`: User exited via prompt input
+- `"other"`: Other reasons
+
+**Example**:
+```yaml
+SessionEnd:
+  - conditions:
+      - type: reason_is
+        value: "clear"
+    actions:
+      - type: output
+        message: "Session cleared - workspace cleanup complete"
+  - actions:
+      - type: command
+        command: "cleanup-session.sh"  # Returns JSON with systemMessage
+```
+
 ## Common Workflows
 
 ### Adding a New Hook Type

README.md

@@ -626,19 +626,19 @@ All conditions return proper error messages for unknown condition types, ensurin
 - `output`
   - Print message
   - Default `exit_status`:
-    - 0 for SessionStart, SessionEnd, UserPromptSubmit (non-blocking events)
+    - 0 for SessionStart, UserPromptSubmit (non-blocking events)
     - 2 for Notification, PreCompact
   - Note: PreToolUse, Stop, PostToolUse, and SubagentStop use JSON output (exit_status ignored)
 
 ### Exit Status Control
 
-**JSON Output Events** (SessionStart, UserPromptSubmit, PreToolUse, Stop, SubagentStop, PostToolUse):
+**JSON Output Events** (SessionStart, UserPromptSubmit, PreToolUse, Stop, SubagentStop, PostToolUse, SessionEnd):
 - Always exit with code 0
 - Control behavior via JSON fields (`decision`, `permissionDecision`, etc.)
 - Errors logged to stderr as warnings
 - See CLAUDE.md for detailed JSON output format
 
-**Legacy Exit Code Events** (Notification, PreCompact, SessionEnd):
+**Legacy Exit Code Events** (Notification, PreCompact):
 - 0
   - Success, allow execution, output to stdout
 - 2