Define pipeline core types and composition helpers

Background:
- Pipeline design in docs/processing-pipeline-design.md.

Scope:
- Implement core pipeline types in pkg/pipeline (Context with VFS, Step, Pipeline, PipelineResult).
- Provide composition helpers for sequential execution and short-circuit behavior.
- Keep steps pure and isolate I/O.

Acceptance criteria:
- Core types exist with docs and tests for basic composition.
- Pipeline execution short-circuits on error while preserving collected results.