Add AST transform plugin system with serializer, built-in plugins, and exec() integration (#114)

* Add AST transform plugin system with serializer, built-in plugins, and exec() integration

Add a plugin-based AST transform pipeline for rewriting bash scripts.
The pipeline parses scripts into an AST, runs transform plugins, and
serializes back to a bash string that can be executed by any shell.

- BashTransformPipeline: standalone typed builder with generic metadata
  accumulation — output can be run by real bash, Docker, SSH, etc.
- Serializer: full-coverage AST-to-bash serializer (commands, words,
  redirections, arithmetic, conditionals, heredocs)
- TeePlugin: wraps commands with tee/stderr capture for output logging
- CommandCollectorPlugin: walks AST to extract all command names
- Bash.registerTransformPlugin: integrated API where exec() auto-applies
  registered plugins and returns metadata in BashExecResult
- Exports all transform types and classes from the package index

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix lint and flaky test

* readme-update

* readme-update

* lint:banned fix

* readme test coverage

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>