feat(dashboard): nav restructure, full context-help coverage, 11 new ref docs

Navigation:
- New order: Overview · Todo · Domains · Repos · Workstreams (collapsible,
  open:false, with atomic sub-entries: Decisions, Tasks, Debt, Extends,
  Dependencies) · Contributions · SBOM · Progress · Reference (collapsible)
- Reference section gains path:/reference landing page; all 18 doc pages
  listed in nav (alphabetical) and in reference.md table

New pages:
- todo.md — Internal / Ecosystem / Third-party todo classification
- dependencies.md — dependency edge table derived from state/summary
- reference.md — Reference landing page with full doc index

New reference doc pages (11):
  contributions, debt, dependencies, domains, extensions, overview,
  repos, tasks, todo + reference (meta) already added previously

doc-overlay.js — lazy bubblehelp tooltip:
- _titleCache Map + _fetchDocTitle(docPath): on first hover of any ?
  button, fetches the target doc page, parses <h1>, sets btn.title
- Native browser tooltip appears exactly on the ? circle on subsequent hover

Context-help wired on all 14 dashboard pages:
- h1 withDocHelp added to: index, todo, domains, repos, tasks, techdept,
  extensions, dependencies (contributions/workstreams/decisions/sbom/
  progress/reference were already wired)
- domains.md + repos.md: added missing withDocHelp import and live-data link
- tasks/techdept/extensions: removed duplicate _h1 const that caused
  SyntaxError: Identifier '_h1' has already been declared

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

dashboard/src/extensions.md

@@ -127,11 +127,11 @@ const _liveEl = html`<div class="live-indicator">
 </div>`;
 withDocHelp(_liveEl, "/docs/live-data");
 
+const _h1 = document.querySelector("#observablehq-main h1");
+if (_h1) { _h1.style.position = "relative"; withDocHelp(_h1, "/docs/extensions"); }
+
 injectTocTop("ep-kpi-box", _kpiBox);
 injectTocTop("live-indicator", _liveEl);
-
-const _h1 = document.querySelector("#observablehq-main h1");
-if (_h1) _h1.style.position = "relative";
 ```
 
 ## By Type & Status