feat(sdd): add Gitea issue templates and SDD CI gate

Adds .gitea/ISSUE_TEMPLATE/{feature,bug}.md (the feature template mirrors the
EARS feature-spec) and .gitea/workflows/sdd-gate.yml — spec-lint, contract
validation (Spectral), traceability check (all non-blocking during adoption),
and a constitution-impact PR comment.

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

.gitea/ISSUE_TEMPLATE/bug.md

@@ -0,0 +1,40 @@
+---
+name: "Bug"
+about: "Something is broken. Describe user-facing impact, not the technical cause."
+title: "<What breaks> when <trigger>"
+labels:
+  - bug
+assignees: []
+---
+
+<!--
+  Title format (COLLABORATING.md): "<What breaks> when <trigger>", e.g.
+  "Upload fails silently when file exceeds 50MB". Keep it focused — a bug is small and direct.
+  A failing test is written first, then the fix (red/green TDD).
+-->
+
+## What happens
+
+<The observed broken behavior, from the user's perspective.>
+
+## Expected
+
+<What should happen instead.>
+
+## Steps to reproduce
+
+1.
+2.
+3.
+
+## Originating requirement (if known)
+
+<REQ-NNN + feature this regresses, from .specify/rtm.md — e.g. "REQ-008 (profile-picture-upload)". Helps target the failing test. Write "unknown" if not traceable.>
+
+## Environment
+
+<Browser / role / data state / deploy (local vs prod) as relevant.>
+
+## Notes
+
+<Logs, GlitchTip link, screenshots. Redact PII.>