refactor: move event system to reducer, remove engine package (#32)

## Description

This PR completes the MVC-inspired refactoring by moving the event system from the engine into the reducer.
The engine package is now removed entirely, as the reducer handles both reduction logic and lifecycle events.

- Add `pkg/reducer/events.go` with `StartEvent`, `StepEvent`, and `StopEvent`.
- Extend `Reducer` interface to embed `Emitter[Event]` and add `Expression()` method.
- Update `NormalOrderReducer` to embed `BaseEmitter` and emit lifecycle events during reduction.
- Update all plugins to attach to `Reducer` instead of `Engine`.
- Remove `internal/engine` package entirely.
- Add `Off()` method to `BaseEmitter` to complete the `Emitter` interface.
- Fix `Emitter.On` signature to use generic type `E` instead of `string`.

### Decisions

- The `Reducer` interface now combines reduction logic with event emission, making it the single orchestration point.
- Plugins attach directly to the reducer, simplifying the architecture.
- The `Expression()` method on `Reducer` provides access to current state for plugins.

## Benefits

- Simpler architecture with one fewer abstraction layer.
- Plugins are now mode-agnostic - they work with any `Reducer` implementation.
- Cleaner separation: reducers handle reduction, plugins observe via events.
- Easier to add new evaluation modes - just implement `Reducer` with embedded emitter.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [x] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #32
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

CLAUDE.md

@@ -80,3 +80,26 @@ Use the `tea` CLI (Gitea command-line tool) for PR operations instead of `gh`.
 
 **Linking issues**: When a PR solves an issue, reference the issue in both the commit message and PR description using `Closes #<number>`.
 This automatically links and closes the issue when the PR is merged.
+
+### Updating PRs
+
+When pushing additional changes to an existing PR, add a comment summarizing the new commits.
+This keeps reviewers informed of what changed since the initial PR description.
+
+Use the `tea` CLI to add comments to pull requests:
+
+```bash
+tea comment <number> "Comment text"
+```
+
+#### Examples
+
+```bash
+# Add a comment to PR #42
+tea comment 42 "Updated implementation based on feedback"
+
+# Add a multi-line comment
+tea comment 42 "Summary of changes:
+- Fixed bug in reducer
+- Added new tests"
+```