feat: write file output, config-relative paths, SGR routing support

[mirpo]

Summary

Adds a terminal write: step and closes the read → LLM → write office loop with no shell glue. Also makes local-file paths config-relative and teaches the schema strict-checker about discriminated unions (SGR routing).

What's in it

• write: step — exports a source step's rows to a deliverable file: .csv (sorted union header, JSON-encoded nested cells), .json (pretty array), .md (table), .jsonl; format inferred from extension or format: override. Terminal (can't be a from/forEach/placeholder source). fs/write.go + step/write_step.go; shared collectRows extracted from transform-collect (no JSONL-plane change).
• Config-relative paths — relative read:/write: paths now resolve to the config file's directory (absolute paths unchanged), so a workflow's data travels with it and runs from any working directory. Applies to both read (was CWD-relative) and write.
• SGR Routing (engine) — the strict-compat walker (jsonschema) now recurses anyOf/oneOf/allOf branches, so discriminated-union schemas are validated for OpenAI strict output. Deterministic tests: TestStrictCompatibilityIssues_UnionBranch, TestValidateJSONText_DiscriminatedUnion.
• Examples — csv-enrichment (minimal read→enrich→write) and inbox-triage (read emails → SGR triage → jq board → draft replies → write csv + md).
• Docs — README "Schema-Guided Reasoning (SGR)" section (Cascade / Routing / Cycle). Routing documented as a cloud-model pattern: small local models (≤3B) reliably mis-route anyOf unions (verified across qwen3:1.7b, llama3.2:3b, deepseek-r1:1.5b), so the local examples use Cascade.

Verification

• Build + full go test ./... + golangci-lint green.
• Both examples run end-to-end from a foreign working directory; deliverables land next to their config (config-relative confirmed live).

Note

Path model has a known open follow-up (intermediate JSONL/dataset/ still resolves CWD-relative while read/write are config-relative; --output control) — tracked separately, out of scope here.