feat(sdd): add .specify scaffold — constitution, AGENTS, personas, templates, example, RTM

Introduces the SDD root: a v1.0.0 constitution and machine-readable AGENTS.md
grounded in the project's real conventions; six EARS-aware persona spec-review
checklists that cross-reference .claude/personas/; feature-spec/ADR/threat-model/
api-contract templates; a fully worked _example feature; a living RTM; and an
adrs/ pointer that reuses the existing docs/adr/ archive.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.specify/adrs/README.md

@@ -0,0 +1,25 @@
+# ADR archive — see `docs/adr/`
+
+This project already keeps a mature, permanent ADR archive at
+[`../../docs/adr/`](../../docs/adr/) (40+ records, format `NNN-kebab-title.md`). SDD does
+**not** introduce a second archive — that would split the project's decision history in two.
+
+## Where ADRs live
+
+- **Project-wide decisions** → [`docs/adr/NNN-kebab-title.md`](../../docs/adr/). Use the
+  next free `NNN` (verify against the directory on disk — parallel worktrees make
+  issue-body numbers stale). Template: [`../templates/adr.md`](../templates/adr.md).
+- **The decision to adopt SDD itself** →
+  [`docs/adr/041-sdd-adoption.md`](../../docs/adr/041-sdd-adoption.md) (this is the
+  "ADR-000" the SDD scaffold calls for, numbered to fit the existing sequence).
+- **Feature-local decisions** that are only meaningful within one in-flight feature →
+  beside that feature's spec, e.g.
+  [`../features/_example/adr-001-avatars-reuse-archive-bucket.md`](../features/_example/adr-001-avatars-reuse-archive-bucket.md).
+  Promote one to `docs/adr/` if its reach turns out to be project-wide.
+
+## Rules (unchanged from the existing convention)
+
+- An ADR is **immutable once `Accepted`** — supersede it with a new, higher-numbered ADR;
+  set the old one's status to `Superseded by ADR-MMM`.
+- Header style matches the existing archive: `# ADR-NNN — Title`, then
+  `**Status:** / **Date:** / **Issue:**`.