docs: output_format: text の使い方をCLAUDE.mdに追記

PreToolUse, PostToolUse, SessionStart, SessionEnd の各セクションに
output_format: text の設定方法と挙動の説明を追加。

Author: syou6162

CLAUDE.md

@@ -297,6 +297,17 @@ SessionStart hooks **always exit with code 0**, even when:
 
 Errors are logged to stderr as warnings, but cchook continues to output JSON and exits successfully. This ensures Claude Code always receives a response.
 
+**Plain Text Output** (`output_format: text`):
+For commands that output plain text (not JSON), use `output_format: text` to treat the stdout as `additionalContext`:
+```yaml
+SessionStart:
+  - actions:
+      - type: command
+        command: "get-project-info.sh"  # Returns plain text, not JSON
+        output_format: text  # stdout treated as additionalContext
+```
+- Without `output_format: text`, non-JSON stdout causes `continue: false` (fail-safe)
+
 **Example**:
 ```yaml
 SessionStart:
@@ -433,6 +444,19 @@ PreToolUse hooks **always exit with code 0**. The `permissionDecision` field con
 
 Errors are logged to stderr as warnings, but cchook continues to output JSON and exits successfully. On errors, `permissionDecision` defaults to `"deny"` for safety.
 
+**Plain Text Output** (`output_format: text`):
+For commands that output plain text (not JSON), use `output_format: text` to treat the stdout as `permissionDecisionReason` with `permissionDecision: "allow"`:
+```yaml
+PreToolUse:
+  - matcher: "Bash"
+    actions:
+      - type: command
+        command: "shellcheck {.tool_input.command}"
+        output_format: text  # stdout treated as permissionDecisionReason, permissionDecision set to "allow"
+```
+- Non-zero exit code still causes fail-safe `permissionDecision: "deny"` behavior
+- Without `output_format: text`, non-JSON stdout causes `permissionDecision: "deny"` (fail-safe)
+
 **Example**:
 ```yaml
 PreToolUse:
@@ -641,6 +665,22 @@ After JSON migration:
 - Use `decision` field instead: omit for allow, `"block"` to prompt Claude
 - A stderr warning is emitted if `exit_status` is set (migration reminder)
 
+**Plain Text Output** (`output_format: text`):
+For commands that output plain text (not JSON), use `output_format: text` to treat the stdout as `additionalContext`:
+```yaml
+PostToolUse:
+  - matcher: "Write|Edit|MultiEdit"
+    conditions:
+      - type: file_extension
+        value: ".go"
+    actions:
+      - type: command
+        command: "gofmt -w {.tool_input.file_path}"
+        output_format: text  # stdout treated as additionalContext, not parsed as JSON
+```
+- Non-zero exit code still causes fail-safe `decision: "block"` behavior
+- Without `output_format: text`, non-JSON stdout causes `decision: "block"` (fail-safe)
+
 **Example**:
 ```yaml
 PostToolUse:
@@ -873,6 +913,17 @@ SessionEnd supports the `reason_is` condition to match session end reasons:
 - `"prompt_input_exit"`: User exited via prompt input
 - `"other"`: Other reasons
 
+**Plain Text Output** (`output_format: text`):
+For commands that output plain text (not JSON), use `output_format: text` to treat the stdout as `systemMessage`:
+```yaml
+SessionEnd:
+  - actions:
+      - type: command
+        command: "cleanup-session.sh"  # Returns plain text, not JSON
+        output_format: text  # stdout treated as systemMessage
+```
+- Without `output_format: text`, non-JSON stdout causes systemMessage set to error message
+
 **Example**:
 ```yaml
 SessionEnd: