Point everything at the repo's new name

[navbytes]

The repo is navbytes/opencode-plugins now, which is what it has been in
practice since it started holding more than one plugin. This updates the 17 lines
that still named the old one.

GitHub redirects the old URL, so nothing was broken meanwhile. Two kinds of
reference do not benefit from that redirect, and are why this lands now rather
than eventually:

• The npm trust commands documented in publish.yml. Trusted publishing
  binds to owner/repo plus the workflow filename, and neither package's record
  exists yet. Creating them against the old name means redoing both — and finding
  out mid-release, when release.yml has already tagged and cut a GitHub release
  and only the OIDC publish fails.
• repository.url / homepage in both packages, which take effect on their
  next publish. Versions already on npm keep the old link, which redirects.

Plus the root README title, the root package name and the matching bun.lock
workspace name (CI runs --frozen-lockfile, so those two have to agree), and the
git-stats test fixtures — including the repo: assertion field, which the URL
rewrite alone would have missed and which would have failed CI.

Deliberately left alone: the five opencode-tree mentions in
packages/context-tree/DESIGN.md are @ishaksebsib/opencode-tree, a different
author's package the design notes compare against; renaming them would have
quietly falsified the comparison. The dated
docs/2026-09-06-roost-hang-report.md also keeps the name things had at the time.

Verified locally: 583 tests pass, typecheck clean, bun install --frozen-lockfile
clean.