feat: dependency edges (native + task-list + body-scrape)

[paolino]

Summary

• Fetch and merge dependency edges from all three sources per Kanban item:
  • Native GitHub issue dependencies — `Issue.blockedBy` / `Issue.blocking`
  • Task-list references — `Issue.trackedIssues` / `Issue.trackedInIssues` (the only signal that spans across orgs)
  • Body keyword scrape — "blocked by #X", "depends on owner/repo#Y", "blocks …", etc., via a small JS regex FFI
• Expanded items now show grouped "Blocked by / Blocking / Tracks / Tracked in" rows. Each edge links to the referenced issue and carries a `[native|tasklist|body]` provenance tag.
• Preview: https://gh-dashboard-edges.surge.sh

Scope not included

• Graph visualisation (dagre / D3). This PR ships the data + inline listing so we can verify coverage on real projects first. Graph view is a follow-up.

Test plan

• Expand an item that has no deps — no new section appears.
• Expand an item with native `Blocked by` links in its sidebar — shows under "Blocked by" with `[native]`.
• Expand an item whose body contains a task-list referencing `owner/repo#N` — shows under "Tracks" with `[tasklist]`.
• Expand an item whose body contains `blocked by owner/repo#N` — shows under "Blocked by" with `[body]`.
• Cross-org task-list references render correctly (different `owner/` prefix).