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/templates/adr.md

@@ -0,0 +1,42 @@
+<!--
+  ADR template. ADRs live in the existing archive: docs/adr/NNN-kebab-title.md.
+  Verify the next free NNN against `ls docs/adr/` on disk (parallel worktrees make
+  issue-body numbers stale). An ADR is IMMUTABLE once Status = Accepted — to change a
+  decision, write a NEW higher-numbered ADR and set this one's Status to Superseded.
+  This header mirrors the existing archive style (see docs/adr/040-*.md). Delete this comment.
+-->
+
+# ADR-NNN — <Short decision title>
+
+**Status:** Proposed <!-- Proposed | Accepted | Deprecated | Superseded by ADR-MMM -->
+**Date:** <YYYY-MM-DD>
+**Issue:** #<n> <!-- the Gitea issue / feature this decision serves -->
+
+## Context
+
+<The forces at play: what problem demands a decision now, the constraints from the
+constitution and existing ADRs, and why the status quo is insufficient. State facts, not
+the chosen answer.>
+
+## Decision
+
+<The decision, stated in active voice as something the project now does. Number sub-decisions
+(### 1, ### 2, …) if the ADR commits several related choices, matching the existing archive.>
+
+## Alternatives Considered
+
+| Option | Pros | Cons | Reason rejected |
+|---|---|---|---|
+| <chosen — name it> | <pros> | <cons> | **Chosen** |
+| <alternative A> | <pros> | <cons> | <why not> |
+| <alternative B> | <pros> | <cons> | <why not> |
+
+## Consequences
+
+<What becomes easier and what becomes harder. Include the obligations this decision places
+on future work (migrations forward-only, tests that must exist, guards that must hold), and
+any new coupling introduced.>
+
+## References
+
+- <constitution §, related ADRs, issue links, external docs>