on_command_completion Lifecycle Event

The current lifecycle events for commands are `on_command` (fires before execution) and `on_command_error` (fires on failure). There is no hook for successful completion, making it impossible to cleanly implement concerns like audit logging, metrics, or post-command cleanup without patching into `after_invoke` on every individual command.

Adding `on_command_completion` to `LIFECYCLE_EVENTS` fills that gap. It would fire after a command finishes without raising an exception, receiving the same `Context` object as `on_command`.

**Proposed API**
```python
@bot.hook
async def on_command_completion(ctx):
print(f"{ctx.command} completed successfully for {ctx.sender}")
```