DAG SVG export to docs + pipeio_docs_build convenience tool

Goal

Write DAG SVGs to disk and build a single convenience MCP tool that chains the full flow docs pipeline.

Context

• mcp_dag_export currently returns DOT/SVG content in-memory — never writes to disk
• docs_collect copies flow docs + notebook outputs to docs/pipelines/<pipe>/<flow>/ but doesn't generate DAG SVGs
• An agent currently has to chain: dag_export → write file → docs_collect → docs_nav → mkdocs_nav_patch
• pipeio_nb_pipeline already exists as a single-notebook convenience chain; this is the flow-level equivalent

Tasks

1. mcp_dag_export: add output_path param with default

• When output_path is set (or defaulted), write SVG to disk and return the path
• Default: docs/pipelines/<pipe>/<flow>/dag.svg
• When output_format="dot" and no output_path, return content only (for piping)

2. docs_collect: auto-generate DAG SVG per flow

• During collection, call dag_export(graph_type="rulegraph", output_format="svg") for each flow
• Write to docs/pipelines/<pipe>/<flow>/dag.svg
• Skip gracefully if snakemake or graphviz not available

3. CLI: pipeio flow dag

• pipeio flow dag <flow> → generates SVG to docs path
• pipeio flow dag <flow> --format dot → stdout (for piping to dot)
• Add to pf.sh as pf <flow> dag

4. MCP: pipeio_docs_build convenience tool

• Single tool that chains: docs_collect (including DAG) → docs_nav → mkdocs_nav_patch
• Optional build_site: bool to also run site_build at the end
• Returns summary of collected files, nav changes, build status

Acceptance Criteria

• [ ] pipeio flow dag ieeg writes docs/pipelines/preprocess/ieeg/dag.svg
• [ ] pipeio docs collect includes DAG SVG per flow
• [ ] pipeio_docs_build() MCP tool chains collect → nav → nav_patch
• [ ] Graceful degradation when snakemake/graphviz unavailable
• [ ] Docs updated (mcp-tools spec, cli reference, how-to)

Result

(Filled in after execution)