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/ep-template.md

@@ -0,0 +1,50 @@
+---
+id: EP-DOMAIN-NNN
+type: ep
+ep_id: EP-DOMAIN-NNN
+target_org: <github-org-or-owner>
+target_repo: <repository-name>
+title: "Short description of the extension point"
+status: draft
+domain: custodian   # change to your domain
+related_workstream: null
+state_hub_contribution_id: null
+created: "YYYY-MM-DD"
+updated: "YYYY-MM-DD"
+location: "src/file.ts:42"
+upstream_issue_url: null
+---
+
+# Extension Point Proposal: EP-DOMAIN-NNN
+
+## Summary
+
+_One-paragraph description of the extension point: where in the upstream codebase
+it would live, what it would enable, and who would benefit._
+
+## Location
+
+**File**: `<path/in/upstream/repo>`
+**Line**: _approximate location where the hook/callback would be inserted_
+
+## Proposed Interface
+
+```typescript
+// or Python, Rust, etc. — match the upstream project's language
+interface ExtensionPoint {
+  // …
+}
+```
+
+## Rationale
+
+_Why this extension point is valuable. What use cases does it unlock?
+Why should upstream accept it rather than keeping it local?_
+
+## Implementation Sketch
+
+_Brief notes on how upstream might implement this, if relevant._
+
+## Notes
+
+_References to related upstream issues, PRs, or discussions._