docs: add architecture decision records and graduate their rules

[gbrlcustodio]

Stack	PR	Base	Content
1 of 2	#597	dev	The living docs: architecture and conventions.
2 of 2	#602 (this one)	docs/architecture-principles	The decision records under docs/contributing/adr/.

Review #597 first. This diff shows only the second step.

What

Record the architecture decisions from the refactor spike as four consolidated decision records, add the guideline that says where a doc belongs, and connect both to the living docs.

• docs/contributing/authoring.md (new): where a doc goes. Audience first (contributor, consumer by surface, durable cross-cutting at root), then Diataxis kind, one kind per file where practical. Generate fast-changing reference from code. Keep recognizable names and do not invent buckets for a few files. It also holds the rule for where a decision record lives.
• docs/contributing/adr/ (new): four records. Layered responsibility (0001), typed single-form contract (0002), MCP tools express outcomes (0003), vertical-slice structure and naming (0004). An index README.md maps each record to the living doc that carries its current rule, and lists the rollout work tracked in refactor: apply the four architecture decisions (ADR rollout) #601.
• docs/mcp/README.md: gains the tool-design section that architecture.md points to.
• docs/contributing/architecture.md: four lines change, each one a back-link to a record this PR creates. That is the whole diff on this file.

All four records land as proposed

None is adopted. A fact-check against the code found that the rules are not yet true of the code, so the set stays one editable proposal rather than a set of immutable records.

This matters for how you read them. An adopted record is immutable, and changing the decision means adding a record that supersedes it. A proposed record is still editable in place, so a correction here is cheap. Say so now if a decision reads wrong.

The rollout that would earn adoption is tracked in #601.

Why the records are separate from the rules

The rule a contributor follows day to day lives in a living doc (architecture.md, conventions.md, mcp/README.md). The record keeps the reasoning that produced it. A living doc can be edited freely as the code moves. A record, once adopted, cannot. Splitting them means the durable why does not rot alongside the current how.

Scope notes

• The ten exploratory records from the spike are consolidated into these four. The full exploration stays in the branch history of refactor/mcp-named-concerns.
• No domain-surface-map.md ships. It derived from an internal model, so its durable slice list moved into ADR-0004 and its migration work-list into refactor: apply the four architecture decisions (ADR rollout) #601.
• Existing record prose is kept intact beyond the targeted status and scope changes, so the recorded reasoning is not altered after the fact.

Testing

Docs-only. No test asserts a docs/ path, and no workflow checks links, so verification is manual:

• Every relative link in the changed files resolves against the tree this PR produces, stacked on docs: consolidate design rationale into contributing architecture and conventions #597 (66 links checked). The four new back-links in architecture.md point at records this same PR adds, so no merge point has a dangling link.
• No code, test, or fixture path changed.

[gbrlcustodio]

Closing in favor of #627, which carries the same branch (docs/decisions) with the correct base.

The content is unchanged and the head commit is the same. The only reason for a new number is that GitHub refuses a base change on a member of a native stack, and this PR still pointed at docs/authoring. After #598 closed, that base was no longer an ancestor of this branch, so the merge base fell back to dev and the diff inflated to 15 files, showing #597's changes as well as this PR's. #627 has the correct base and shows the intended 10 files.

This PR also now includes the authoring guideline from #598 and the record statuses from #625, so the four-PR stack is two PRs: #597 then #627.