docs(spec): align checkpoint trigger names and config key path with implementation

- Update trigger names from 'on_tool_write' and 'on_tool_write_complete' to 'before_tool_execute' and 'after_tool_execute' to match the actual implementation in config_service.py
- Correct config key path from 'core.checkpoints.auto_create_on' to 'checkpoints.auto_create_on' to match the actual configuration structure
- Update TOML configuration examples to use comma-separated string format instead of array format, matching the actual ConfigService implementation
- Update references to trigger names in the CLI Usage section to use the correct names

docs/reference/checkpointing.md

@@ -61,28 +61,28 @@ The rollback operation enforces two guards:
 ## Automatic Checkpoint Triggers (v3.8.0+)
 
 The execution engine now supports four automatic checkpoint triggers, all
-configurable via `core.checkpoints.auto_create_on` (default: all enabled):
+configurable via `checkpoints.auto_create_on` (default: all enabled):
 
 | Trigger | When | Component |
 |---------|------|-----------|
-| `on_tool_write` | Before each write-tool execution | `ToolRunner` |
-| `on_tool_write_complete` | After each write-tool execution | `ToolRunner` |
+| `before_tool_execute` | Before each write-tool execution | `ToolRunner` |
+| `after_tool_execute` | After each write-tool execution | `ToolRunner` |
 | `on_subplan_spawn` | Before first subplan execution attempt | `SubplanExecutionService` |
 | `on_error` | When the Execute phase fails | `PlanExecutor` |
 
 ### Configuration
 
 ```toml
-[core.checkpoints]
-auto_create_on = ["on_tool_write", "on_tool_write_complete", "on_subplan_spawn", "on_error"]
+[checkpoints]
+auto_create_on = "before_tool_execute,after_tool_execute,on_subplan_spawn,on_error"
 ```
 
 To disable a specific trigger, remove it from the list. To disable all
 automatic checkpoints:
 
 ```toml
-[core.checkpoints]
-auto_create_on = []
+[checkpoints]
+auto_create_on = ""
 ```
 
 ### Wiring
@@ -103,7 +103,7 @@ When `checkpoint_service` is `None`, all automatic checkpointing is skipped
 Checkpoints are created automatically by the execution engine via the four
 triggers above, or manually via `CheckpointService.create_checkpoint()`.
 The `CheckpointScope` on each tool's capability declaration controls which
-tools trigger `on_tool_write` / `on_tool_write_complete` checkpoints.
+tools trigger `before_tool_execute` / `after_tool_execute` checkpoints.
 
 ### Rollback to a checkpoint