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._

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

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+---
+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._

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._

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

@@ -0,0 +1,62 @@
+---
+id: upr-YYYY-MM-DD--org--repo--slug
+type: upr
+target_org: <github-org-or-owner>
+target_repo: <repository-name>
+title: "Short description of the upstream PR"
+status: draft
+domain: custodian   # change to your domain
+related_workstream: null
+state_hub_contribution_id: null
+created: "YYYY-MM-DD"
+updated: "YYYY-MM-DD"
+local_component_path: "relative/path/in/this/repo/component.js"
+target_upstream_file: "path/in/upstream/repo/component.js"
+upstream_pr_url: null
+---
+
+# Upstream PR: <short description>
+
+## Summary
+
+_What this PR adds or fixes in the upstream project._
+
+## Motivation
+
+_Why we want this upstream rather than keeping it as a local fork.
+What maintenance burden does upstreaming remove?_
+
+## Local Component
+
+**Path in this repo**: `<local_component_path>`
+**Origin story**: _How/when this component was created locally._
+
+## Target Upstream Location
+
+**Repo**: `<target_org>/<target_repo>`
+**File**: `<target_upstream_file>`
+
+## Draft PR Body
+
+```markdown
+## Summary
+
+_Fill in the PR summary for the upstream project here._
+
+## Motivation
+
+_Explain why the upstream project would benefit._
+
+## Changes
+
+- …
+
+## Testing
+
+- [ ] Tests added
+- [ ] Existing tests pass
+```
+
+## Notes
+
+_Any upstream discussion, review feedback, or open questions._