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/fr-template.md

@@ -0,0 +1,40 @@
+---
+id: fr-YYYY-MM-DD--org--repo--slug
+type: fr
+target_org: <github-org-or-owner>
+target_repo: <repository-name>
+title: "Short description of the requested feature"
+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_issue_url: null
+---
+
+# Feature Request: <short description>
+
+## Motivation
+
+_Why we need this feature. What problem does it solve? What becomes possible?_
+
+## Proposed Interface / API
+
+```
+# Sketch of the proposed interface, function signature, or configuration option.
+```
+
+## Example Usage
+
+```
+# How we would use this feature in our codebase once available.
+```
+
+## Alternatives
+
+_What we currently do instead (workaround, local implementation, etc.)._
+
+## Notes
+
+_Any additional context, links to related discussions, or upstream roadmap references._