Decision records get a current-design layer and a domain index #726

docs/decisions/ holds 44 records (about 4,500 lines), flat and numbered in creation order. One topic is spread across many numbers (ontology: 0001, 0003, 0006–0012, 0036, 0044; time: 0019, 0022, 0024, 0031, 0045; governance: 0017, 0025–0028, 0043). Knowing what the current design is means reading the history, because later records revise earlier ones in part and the index status lines have grown into paragraphs. A file name says when a record was written, not what it is about or whether it still holds.

Proposal

docs/
  README.md        index by domain: current design and history; each record with date,
                   title and one status word (current, partly superseded, superseded);
                   a table from old numbers to new paths
  design/          current design, one file per domain, under 150 lines each:
                   what it does, why, links to the evidence, open questions
  decisions/       history, one subdirectory per domain, files named
                   YYYY-MM-DD-short-title.md, contents unchanged, original number in the header

Domains (to confirm after reading each record): ontology, extraction, identity, ledger and time, governance and review, rules and reasoning, sources and ingestion, lakehouse and actions, access and audit, interface, chat and MCP.

Conventions change with it: a design file changes when the design changes; a decision record records why, and is never rewritten (the existing revision-in-place rule). Code comments referencing numbers (about 40 sites) stay valid through the index table and are updated as touched.

Sequence

After #699, #710 and #724 settle, in one documentation PR. Every record is read before it is filed; titles alone misfile some (0014 is about access, not identity).