Part 2 of hyperhive#1970 (backend/CLI landed in PR #2378). Renames
/matrix-accounts.html -> /credentials.html and restructures it with a
sub-tab strip (MATRIX / GITHUB), reusing the shared @hive/shared/tabs.js
tab strip already used by /logs.html.
MATRIX tab carries over the existing account list + login form
unchanged. GITHUB tab adds a single-PAT provisioning form: status line
(present/absent, read from GET /api/github-account), a security-warning
banner (dedicated bot account + minimally-scoped token), a link to
generate a PAT at github.com/settings/tokens, and a paste-token form
posting to POST /api/github-account. Both tabs share one agent picker.
Updated docs/web-ui.md + docs/web-ui/dashboard.md to describe the new
page shape, and the H0M3 hub tile (index.html) to point at the renamed
page.
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