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>

contrib/upstream-prs/2026-02-26--observablehq--framework--toc-sidebar-inject.md

@@ -0,0 +1,99 @@
+---
+id: upr-2026-02-26--observablehq--framework--toc-sidebar-inject
+type: upr
+target_org: observablehq
+target_repo: framework
+title: "Add injectTocTop() utility for injecting elements into the TOC sidebar"
+status: draft
+domain: custodian
+related_workstream: contribution-tracking-sbom
+state_hub_contribution_id: null
+created: "2026-02-26"
+updated: "2026-02-28"
+local_component_path: "state-hub/dashboard/src/components/toc-sidebar.js"
+target_upstream_file: "src/client/toc.ts"
+upstream_pr_url: null
+---
+
+# Upstream PR: Add injectTocTop() utility for injecting elements into the TOC sidebar
+
+## Summary
+
+Propose adding an `injectTocTop(id, element)` utility to Observable Framework
+that allows dashboard pages to prepend arbitrary HTML elements at the top of the
+automatically-generated table-of-contents sidebar. This enables KPI cards, live
+status indicators, and other contextual widgets to live alongside the page's
+section links.
+
+## Motivation
+
+Observable Framework generates a TOC sidebar from headings automatically, but
+provides no standard way to inject content into it from page JavaScript. As a
+result, every project that wants sidebar widgets must implement its own DOM
+manipulation workaround.
+
+By upstreaming a small, focused utility, the community gets a stable API for
+sidebar injection that doesn't depend on Observable Framework's internal DOM
+structure.
+
+## Local Component
+
+**Path in this repo**: `state-hub/dashboard/src/components/toc-sidebar.js`
+**Origin story**: Created 2026-02-26 during implementation of the Decisions KPI
+dashboard (State Hub v0.2). The component provides two utilities:
+- `injectTocTop(id, element)`: remove-then-prepend to the TOC aside element
+- `withDocHelp(element, docPath)`: attaches a `?` button that opens a doc page
+  as an iframe overlay
+
+## Target Upstream Location
+
+**Repo**: `observablehq/framework`
+**File**: `src/client/toc.ts` (or a new `src/client/toc-inject.ts`)
+
+## Draft PR Body
+
+```markdown
+## Summary
+
+Add `injectTocTop(id, element)` — a small utility for injecting custom elements
+at the top of the Observable Framework table-of-contents sidebar.
+
+## Motivation
+
+Dashboard pages frequently need contextual widgets (KPI boxes, live status
+indicators, navigation aids) that belong alongside the TOC, not in the page
+content flow. Currently this requires each project to directly manipulate the
+`#observablehq-toc` DOM element, coupling page code to an internal DOM detail.
+
+`injectTocTop` provides a stable API for this pattern:
+
+```js
+import {injectTocTop} from "observablehq:toc";
+
+const myWidget = html`<div class="kpi-box">...</div>`;
+injectTocTop("my-kpi-box", myWidget);
+```
+
+## Changes
+
+- Add `injectTocTop(id: string, element: HTMLElement): void` to Framework's
+  client runtime
+- Export from the `observablehq:toc` module (or equivalent public API)
+- Add documentation page: "Customising the sidebar"
+
+## Testing
+
+- [ ] Unit test: `injectTocTop` prepends element to `#observablehq-toc aside`
+- [ ] Unit test: calling again with same `id` removes old element before
+  inserting new one (idempotent)
+- [ ] Integration test: injected element appears above TOC links in rendered page
+```
+
+## Notes
+
+- The local implementation (`toc-sidebar.js`) uses `querySelector('#observablehq-toc aside')`
+  — this should be replaced with a stable API reference once upstreamed.
+- The `withDocHelp` utility in the same file is a separate feature; this PR
+  focuses solely on `injectTocTop`.
+- Observable Framework issue tracker should be checked for any existing
+  discussion of sidebar customisation before opening a new issue.