Add docs/README.md — human reading order / docs index

> _Filed by Claude (AI assistant) on @jmchilton's behalf — not personally authored._

## Gap

`AGENTS.md` defines a reading order for agents ("Read first / Then orient"), but there is no equivalent landing page for a human arriving at `docs/` cold. There are 9 docs in `docs/` (`ARCHITECTURE.md` 559 lines, `COMPILATION_PIPELINE.md`, `MOLD_SPEC.md`, `MOLDS.md`, `GUIDING_PRINCIPLES.md`, `HARNESS_PIPELINES.md`, `CORPUS_INGESTION.md`, `PATTERNS.md`, `SCHEMA_PACKAGES.md`) with no map of which to read for which question.

## Proposed fix

A short `docs/README.md`: one line per doc, grouped by the question it answers ("what is this / why" → README + GUIDING_PRINCIPLES; "how is it structured" → ARCHITECTURE; "how do I author a Mold" → MOLD_SPEC + MOLDS; "how does casting work" → COMPILATION_PIPELINE; "how is the corpus used" → CORPUS_INGESTION). Mirror, don't duplicate, the `AGENTS.md` ordering — link to it rather than restate.

## Notes

- Pure navigation; no content migration. Independent of any grant work.
- Effort: low.
- Surfaced during a review of transient grant-proposal hardening notes; this is the durable, grant-independent residue.