## Why

Current workflow engines force a tradeoff between simplicity and durability. Archon has a clean DAG-based YAML format but no crash recovery. Vercel Workflow has bulletproof deterministic replay but requires a Rust compiler plugin and 24-36 weeks to build. Agent SOP proves that human-readable markdown workflows work, but lacks structured execution. There is no portable workflow engine that combines a simple DAG format, human-readable definitions, and durable event-sourced execution in a single, buildable package.

## What Changes

Introduce **Ion** — a portable hybrid workflow engine that combines the three approaches:

- **Archon-style YAML DAG format** with `nodes:`, `depends_on:`, and trigger rules as the primary workflow definition
- **Agent SOP-style `.sop.md` markdown** as a secondary human-readable format, auto-converted to the DAG representation
- **Vercel-style event log** for deterministic replay and crash recovery, but simplified — no SWC plugin, no VM sandbox, no compile-time transforms
- **Multi-backend storage** (filesystem for dev, SQLite/Postgres for production)
- **CLI + library** dual distribution: use as a CLI tool or embed as a library
- **No Rust compiler plugins, no SWC, no VM sandbox** — pure TypeScript/JavaScript, zero compile-time transforms

## Capabilities

### New Capabilities

- `dag-engine`: DAG execution engine with topological ordering, concurrent layers, trigger rules (`all_success`, `one_success`, `all_done`, `none_failed_min_one_success`), and `when:` condition evaluation
- `yaml-format`: Workflow definition in YAML with 7 node types (command, prompt, bash, script, loop, approval, cancel) plus `depends_on`, `trigger_rule`, `output_format`, `retry`, `timeout`
- `markdown-format`: `.sop.md` human-readable workflow format with RFC 2119 constraint keywords, auto-converted to DAG nodes
- `event-sourcing`: Append-only event log for workflow runs with deterministic replay for crash recovery — simplified (no SWC, no VM sandbox)
- `variable-substitution`: `$nodeId.output`, `$nodeId.output.field`, `$ARGUMENTS`, `$ARTIFACTS_DIR`, `$WORKFLOW_ID`, `$LOOP_PREV_OUTPUT`, `$REJECTION_REASON` with strict field access
- `script-execution`: Script node type running TypeScript (bun/node) and Python (uv/python3) with `deps:` support and `timeout:`
- `human-approval`: Approval gate nodes that pause execution for human review with `capture_response` and `on_reject` retry support
- `storage-backends`: Pluggable storage — filesystem (dev), SQLite, Postgres — with IWorkflowStore interface
- `workflow-lifecycle`: Run states `pending → running → paused/completed/failed/cancelled`, resume skipping completed nodes, event-driven observability
- `cli-tool`: Command-line interface for listing, running, approving, rejecting, resuming, and cleaning up workflow runs
- `library-api`: Programmatic API for embedding the engine in other applications

### Modified Capabilities

<!-- No existing specs to modify — this is a greenfield change. -->

## Impact

- **Greenfield project** — no existing code to modify, all new artifacts under `ion/` or equivalent package path
- **Dependencies**: Zod (schema validation), nanoid/ulid (ID generation), js-yaml (YAML parsing), chokidar (file watching for dev mode)
- **Optional dependencies**: better-sqlite3 / postgres.js (production storage backends), bun (fast script runtime), highlight.js (markdown rendering)
- **No** Rust, SWC, wasm, or compile-time transforms in the core engine