refactor(sdd): make the feature spec issue-only (no committed spec.md)

The Gitea issue body is the single source of truth for a spec; the only
per-feature artifact in git is the RTM row (REQ-ID -> issue # -> test). Drops
per-feature spec.md/tasks.md/checklist files from the workflow (the _example
stays as a template/reference). Updates the guide, ADR-041, AGENTS.md, CLAUDE.md,
templates, the RTM (adds an Issue column), the implement/review-pr skills, and
replaces the file-spec CI jobs with an rtm-check.

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

.gitea/ISSUE_TEMPLATE/feature.md

@@ -9,10 +9,9 @@ assignees: []
 ---
 
 <!--
-  This is a Spec-Driven feature spec. Mirror it to .specify/features/<name>/spec.md once the
-  feature has a working name. Every requirement uses an EARS pattern + a REQ-NNN id.
-  Reference: .specify/templates/feature-spec.md and the worked example .specify/features/_example/.
-  Delete the placeholder hints as you fill each section.
+  This issue body IS the spec (issue-only — there is no committed spec.md). Every requirement
+  uses an EARS pattern + a REQ-NNN id. Reference: .specify/templates/feature-spec.md and the
+  worked example .specify/features/_example/. Delete the placeholder hints as you fill each section.
 -->
 
 ## Context & Why
@@ -46,7 +45,7 @@ assignees: []
 
 ## API / Contract Stub
 
-<Inline stub or link to .specify/features/<name>/api-contract.yaml. Name new paths/methods/status codes and the @RequirePermission on each mutating endpoint.>
+<Inline OpenAPI stub (use .specify/templates/api-contract-stub.md as a writing aid). Name new paths/methods/status codes and the @RequirePermission on each mutating endpoint.>
 
 ## Data Model Changes