feat(canon): add contribution-convention v0.1, contrib/ templates, and first UPR artifact

- canon/standards/contribution-convention_v0.1.md: master spec for BR/FR/EP/UPR
  artifact types, directory layout, frontmatter schema, ID schemes (EP-DOMAIN-NNN
  for extension points), status lifecycle, and relationship to State Hub
- canon/standards/contrib-templates/: four template files (br, fr, ep, upr)
- contrib/upstream-prs/2026-02-26--observablehq--framework--toc-sidebar-inject.md:
  first real UPR artifact — proposes injectTocTop() to Observable Framework

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canon/standards/contrib-templates/br-template.md

@@ -0,0 +1,44 @@
+---
+id: br-YYYY-MM-DD--org--repo--slug
+type: br
+target_org: <github-org-or-owner>
+target_repo: <repository-name>
+title: "Short description of the bug"
+status: draft
+domain: custodian   # change to your domain
+related_workstream: null
+state_hub_contribution_id: null
+created: "YYYY-MM-DD"
+updated: "YYYY-MM-DD"
+upstream_version: "X.Y.Z"
+upstream_issue_url: null
+---
+
+# Bug: <short description>
+
+## Environment
+
+- **Upstream project**: `<target_org>/<target_repo>` v`<upstream_version>`
+- **Our usage context**: _which component or module triggered this_
+
+## Reproduction Steps
+
+1. …
+2. …
+3. …
+
+## Expected Behaviour
+
+_What should happen._
+
+## Actual Behaviour
+
+_What actually happens. Include error messages or stack traces._
+
+## Workaround
+
+_Any workaround currently in use, or "None"._
+
+## Notes
+
+_Any additional context, links to related issues, or observations._